Answer:
The only thing you will do is to plug the x in the equation.
x = -2 ---> f(x) = -6
x = 0 ---> f(x) = 0
x = 3 ---> f(x) = 9
x = 7 ---> f(x) = 21
f(x) = 3x
f(-2) = 3(-2)
f(-2) = -6
f(0) = 3(0)
f(0) = 0
f(3) = 3(3)
f(3) = 9
21 = 3x
21 / 3 = 3x / 3
7 = x
Hope this helps, thank you :) !!