Answer: ~96.83°
Explanation:
1) It's obvious that the largest angle is the opposite to the largest side.
▪︎ QR = 18 (the largest side)
=> <QPR is the largest angle
2) Cosine Rule:
▪︎ QR^2 = QP^2 + PR^2 - 2*QP*PR*cos(QPR)
▪︎ 18*18 = 13*13 + 11*11 - 2*13*11*cos(QPR)
▪︎ 324 = 169 + 121 - 286*cos(QPR)
▪︎ 34 = -286*cos(QPR)
▪︎ cos(QPR) = -34/286
▪︎ <QPR = arccos(-34/286) = 96.827...°