Question

If g(x) = 2(x − 4), find the value of x if g(x) = 20.

Answer 1

$g(x)=2(x-4)\\\\g(x)=20\to2(x-4)=20\qquad\text{divide both sides by 2}\\\\x-4=10\qquad\text{add 4 to both sides}\\\\\boxed{x=14}$

Answer 2

Here you're given a numerical value for g(x) and an algebraic formula for g(x) also.  Equate the two:  g(x) = 2(x - 4) = g(x) = 20.  

Solve this for x:

Divide both sides by 2.  We get:  x - 4 = 10.

Add 4 to both sides, to isolate x:  x = 14.

The value of x is 14.