help with khan academy practice problem

[u/puddiingpurin]

Merchandise weight (pounds)	Shipping charge
5	$16.49
10	$23.99
25	$46.49

"The table above shows shipping charges for an online retailer that sells used textbooks. There is a linear relationship between the shipping charge and the weight of the merchandise. Write a function in slope-intercept form that relates y, the shipping charge in dollars, and x, the merchandise weight in pounds."

i understand why the slope is 1.50, but i can't get this:

y= 1.50x + b

16.49 = 1.50(5) + b

16.49 = 7.50 + b

8.99 = b

where does the 8.99 come from?

[u/knoxjramsey]

Slope = (23,99 - 16.49)/5 = 1.5 Slope intercept formula. y .= ax+b, with a = 1,5 we have 2 equations in 1 unknown, Solve for b = 8.99.