Answer
$$x=3a$$
Work Step by Step
We're given
$$x-a=2(x-2a)$$
Distributing scalar multiplication over vector addition on the right hand side, we get:
$$x-a=2x-4a$$
By subtracting $2x-a$ from both sides, we then move all the $x$s to the left side and all the $a$s to the right:
$$-x=-3a$$
Lastly, dividing both sides by $-1$ yields
$$x=3a$$
