Answer:
[tex] x = 2 [/tex]
Step-by-step explanation:
We need to find the value of x so that the value of given function becomes 21 .
We have ,
[tex]f(x)=21\\\\ f(x)= 6x + 9 [/tex]
Now , for finding x ,
[tex] 6x +9 = 21 \\\\ 6x = 21 -9 \\\\ 6x = 12 \\\\ x = 2 [/tex]
