Question

Y=f(x)=-4x find f(x) when x=-3

Answer 1

F(x)=-4x                 x=-3                          Y=f(x)

replace -3 for x so it would then look like:      f(x)=-4(-3)
Distribute -4 to -3
Answer is f(x)=12

Answer 2

So your equation is f(x)=-4x   

If your x=-3, you can rewrite your equation as 

f(x)= -4(-3)

To find your answer, simply multiply -4 x -3 and your answer should come to an answer of -12