Question

What is the equation of a line that passes through (7,8) and has a slope of -3

Answer 1

It is helpful to first plug this in to point-slope form (note that there are other ways to do this).

Using the form $y-y_1 = m(x-x_1)$, you get $y-8=-3(x-7)$. You can simplify this to get $y = -3x+29$.

Answer choice A

Answer 2

You use y=mx+b, then substitute -3 for the slope (m) and 7 for x and 8 for y, after you substitute all those things - use that equation to solve for the b-term - that will give you the y-intercept. Then you will rewrite the equation of the line as y=-3x+(whatever you just found for b).