Is it a parallelogram? If yes then, I believe there's a property that two triangles on the base of a ||gm are equal in area, and precisely half the area of the ||gm.

So by that property,

=> 143 + Middle + 6 = 136 + Middle + ?

=> 143 + 6 = 136 + ?

=> 149 - 136 = ?

=> ? = 13

ETA : 136* and 13*