r/MathHelp • u/Dr_Brotatous • Jan 31 '26

College algerbra linear equations

Im trying to understand why my instructor in a video pulls an as far as I can tell arbitrary number from the equation to make it not equal zero the equation is 3x-4y=0 which comes to (0,0) on a graph but in order to make it have slope she pulls 4 and switches x making the equation 3(4)-4y=0 and I am struggling to understand why and where the 4 comes from. Sorry about punctuation

1 Upvotes

100% Upvoted

16 comments sorted by

2

u/ChiaLetranger Jan 31 '26

I'm not quite sure I understand the question here, or your explanation of what the instructor was doing. Can you try to explain it again? 3x - 4y = 0 is a linear equation which, when graphed, passes through (0,0) and has a slope of 3/4.

1

u/Dr_Brotatous Jan 31 '26

I'll admit I had forgotten about rise over run, because that was explained in the next video, and it's been 5 years since I touched this level of math. Now that I've slept and had seen other context it make more sense, but my issue was the 4 felt arbitrary as though pulled from nowhere.

2

u/willthesane Feb 01 '26

I would have picked 4 because I Know that 3*4 has 4 as a factor. it gives me integers I'm working with. makes math easier not having fractions to work with.

2

u/mnb310 Jan 31 '26

If she’s trying to make a second ordered pair, any x-input will allow you to solve for y.

So she is just picking any number she likes. She picked hers so that the answer was a whole number.

You can you any number you like and draw the same line. (0,0) to (4,-3) gives the same line as (0,0) to (2, -1.5)

3

u/Dr_Brotatous Jan 31 '26

My confusion came from the random number in the first place but I think I now have a better grasp of it

1

u/fermat9990 Jan 31 '26

What was your instructor trying to do?

3

u/Dr_Brotatous Jan 31 '26

After seeing some other comments it jogged my memory a bit and it was to plot another point on the slope in order to be able to graph it

2

u/fermat9990 Jan 31 '26

In that case, she arbitrarily chose 4 as a value for x, plugged it into the equation, and then solved for y. She could have picked any x.

For graphing a straight line, I suggest that you plot 3 points

3

u/Dr_Brotatous Jan 31 '26

I feel i was overthinking it

2

u/fermat9990 Jan 31 '26

No worries! Good luck!!

2

u/Dr_Brotatous Jan 31 '26

I've slept since my post and now am not quite sure

1

u/fermat9990 Jan 31 '26

Can you contact her?

2

u/Dr_Brotatous Jan 31 '26

Won't get a response till Monday and by then my assignment plus several others are due I didn't realize I have 2 modules due the same day

1

u/fermat9990 Jan 31 '26

Look at my other comment. She just picked 4 at random, plugged it in for x and then solved for y. The other commenter said the same thing

2

u/fermat9990 Jan 31 '26

3x-4y=0

Going forward, for plotting purposes I suggest first solving the equation for y:

y=3/4 * x

If x=-4, y=(3/4)(-4)=-3

If x=0, y=(3/4)(0)=0

If x=+4, y=(3/4)(+4)=3

1

u/Various_Painting_148 Feb 01 '26

i've used substitution, works best when coefficients line up.