-2
Step-by-step explanation:
Put the given value in the equation and solve for x.
 f(x) = 12
 f(x) = 2(x +8)
 12 = 2(x +8) . . . . . use 12 for f(x)
 6 = x +8 . . . . . . . . divide by 2
 -2 = x . . . . . . . . . . subtract 8
-14
step-by-step explanation:
2x2 – 2(x+4)
=2x2-2(5+4)
=4-2×9
=4-18
=-14