Question

How do I work this problem?
m=-3, (4,5)

Answer 1

I gues you are looking for a straight line of slope m = 3 through the point (4,5)

y = 3 x + b

5 = 3*4 + b
5 = 12 + b
b = -7

so
y = 3 x - 7

Answer 2

or, since you have a point and a slope, you can just use the point-slope form:

y-5 = 3(x-4)