The answer would be x=-4.
12x - 18 - 4x -2
12x - 4x = 8x (yours 12x - 4x = 8)
-18 - 2 = -20 (yours -18 -1 = 17)
answer:
8x - 20
(a ± b)² = a² ± 2ab + b² . . . . . . . (signs match)
The middle term is twice the product of the roots of the other two terms. This tells you the terms of the binomial are the square roots of the end terms.
The sign in the binomial will match the sign of the "2ab" term. The order of terms in the binomial doesn't matter. (a±b)² = (b±a)² when signs match.
I don't get how it would be a system of equations.
By definition, we have
So, we have to solve two different equations, depending of the possible range for the variable. We have to remember about these ranges when we decide to accept or discard the solutions:
Suppose that 
In this case, the absolute value doesn't do anything: the equation is
We are supposing 
, so we can accept this solution.
Now, suppose that 
. Now the sign of the expression is flipped by the absolute value, and the equation becomes
Again, the solution is coherent with the assumption, so we can accept this value as well.
