That's an isosceles triangle lying on its side. The base angles (on the left) are marked equal, so we know the sides opposite them are equal.
That means that . . . x+4 = 3x-8
Subtract 'x' from each side . . . 4 = 2x-8
Add 8 to each side . . . 12 = 2x
Divide each side by 2 . . . 6 = x