It is an isosceles trapezoid, and so it's two leg sides would be congruent.
AC ≅ BD
Set the two measurements equal to each other
3x - 12 = 2x + 4
Isolate the x. subtract 2x from both sides and add 12 to both sides (note equal sign. What you do to one side, you do to the other.)
3x(-2x) - 12 (+12) = 2x (-2x) + 4 (+ 12)
3x - 2x = 4 + 12
x = 16
16 is your answer for x
hope this helps
