Question

find the equation of the line having the given slope and containing the given point
m=-3(1,5)

Answer 1

y = -3x + 8

Answer 2

if the slope is -3 then your equation is y = -3x + b The you substitute in for (1,5)
5 = -3(1) +b
b = 8
Therefore you have: y = -3x + 8

Answer 3

and you can check your result by plugging in 1 for x and indeed you do get 5. So that point defines the line.