if AB=CD, AB is 12-y, CD = 3x-4y, BD = 18, AC = x+y, what is x, y, and AC

Answer 1

No way to tell, unless ABCD is a parallelogram. In that case, AB=CD and AC=BD

12-y = 3x-4y
x+y = 18

(x,y) = (11,7)