x=-6
Explanation
to solve this, let´s complete the tables and then compare
Step 1
complete the table:
to do that we need to input the x value and evaluate to obtain f(x)
so
a) for

i)when x=-12

ii) when x= -9

iii) when x=-6

iv) when x= -3

hence
Step 2
Now , for equation(2) g(x)

a) when x=-12

b) when x=-9

c) when x=-6

d)when x=-3

so
Step 3
finally, compare and check wich input value produces the same out put
therefore, the answer is
x=-6
I hope this helps you