To find how many degrees a polygon's angles add up to, use the formula (n - 2) · 180, where n = the number of angles. So, since this polygon has 5 angles (it's a pentagon)
(5-2) · 180 = 3 · 180 = 540.
Now, all we have to do is add up all of the angles:
75 + 126 + 126 + 81 + c = 540
c = 132
