You could increase your chances of getting the help you need and want if you'd please be sure to include the instructions for each problem.
y = -4.5 + 15 would be a linear function whose rate of change (slope) is -4.5 if you had included the "x" after -4.5: y = -4.5x + 15. Compare this to
y = mx + b
Seems as tho' you have shared five possible "solutions." None of them would satisfy y = -4.5x + 15.
I'd suggest you go back to the original problem and ensure that you've copied it down here exactly as it was presented there.