أولاً نبسّط الأساس: (86+3) = 89. Reduce it modulo 7: 89 ≡ 5 (mod 7). The powers of 5 modulo 7 repeat in the cycle 5, 4, 6, 2, 3, 1. Since exponent 91 points to the cycle value 5, 89^91 leaves remainder 5.