Respuesta :

tramserran
The diagonals of a parallelogram bisect each other so AE = AC:
     AE   =   AC
3x + 12 = 5x - 6
        12 = 2x - 6
        18 = 2x
           9 = x

AE = 3x + 12
     = 3(9)+12
     = 27 + 12
     =    39

AC = AE = 39

Use the Segment Addition Theorem:
AE + AC = AC
 39 + 39  = AC
     78      = AC

Answer: AC = 78