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.
