In this case, we'll have to carry out several steps to find the solution.
Step 01:
y = 2(x-a)
x = ?
Step 02:
We must apply the algebraic properties to find the solution.
y = 2(x-a)
y = 2x - 2a
y + 2a = 2x
(y + 2a) / 2 = x

The answer is:
x = (y + 2a) / 2