In the figure below, if BE is the perpendicular bisector of AD, what is the value of x?

Both sides are the same, set them to equal and solve for X.
3X +4 = X +12
Subtract 4 from each side:
3X = x + 8
Subtract 1X from each side:
2X = 8
Divide both sides by 2:
X = 8/2
X = 4