You can get all the terms on one side, and then solve for x by any means:

Factor out a -2:

Factor this equation:

I will use the AC method. To use it, first multiply a and c (in ax^2 + bx +c):

Now, look for two numbers that multiply to -12 and add to -11. Obviously these numbers are -12 and 1. (-12*1 = -12 and -12+1 = -11). Now, because of rules, you set it up in a Punnett square:
4x^2 -12x
x -3
Now, we find common factors of the terms in rows:
_x_______-3__
4x| 4x^2 -12x
1 | x -3
So, you can use this to write an equivalent expression to the quadratic given:

Now, we known the factors are (solve for x):
3 and -1/4.
Take the square root of 256 to find the side length of the mirror
√256 =16
the rectangles width is 16, it length is 2 inches longer
16+2 =18 so the length is 18
to find the area multiply them together
18x16=288
the area is 288 inches² :)