Answer:
x=5
Step-by-step explanation:
F(x)=-3x+21,
f(x)=6 = -3x+21
6 = -3x+21
Subtract 21 from each side
6-21 = -3x+21-21
-15 = -3x
Divide each side by -3
-15/-3 = -3x/-3
5 = x
