88 + 99 + (n-2)*163 = (n-2)*180 88 + 99 + 163n - 2*163 = 180n - 2*180 88 + 99 - 2*163 + 2*180 = 180n - 163n 221 = 17n n = 221/17 = 13. ANSWER. 13 sides.