Answer:
x = 5
Step-by-step explanation:
We have,
[tex]f(x) = -3x + 21[/tex] .....(1)
It is required to find x when the value of f(x) = 6
Put f(x) = 6 in equation (1) such that we get :
[tex]6 = -3x + 21\\\\3x=21-6\\\\3x=15\\\\x=5[/tex]
The value of x is 5 if f(x) = 6.
