so the exterior angle of any polygons are 360 in total. since in the pic there r some interior angles, we need to 180 - interior to get the exterior angle. since they're adjacent angles on a straight line.
so therefore we get
51 + 66 + x + 180 - 132 + 78 + 180 - 139 = 360 (sum of exterior angle of polygon)
284 + x = 360
x = 76°
so it's C.