亦解

与monseigneur的解法类似，利用相似三角形带来的比例性质
 18/(x+18) = S_AEF/S_ACD = (AE*AF)/(AC*AD)
   = AE/(AE+EC) * AF/(AF+FD)
   = AG/(AG+BC) * AG/(AG+BD)
   = (BH/2) / (BH/2+BH/3)  * (BH/2)/(BH/2 + 2*BH/3)
   = 9/35
 因此
 18/x = 9/(35-9) = 9/26
 x = 52