Note that only 5, 7, and 9 are used, and so just solve for those
Replace n with {5}
(5 x 5)² + 5 = 25² + 5 = 225 + 5 = 330
(9 x 9) - 5 = 81 - 5 = 76
330/76 = ~4.34 < 28 (True)
Replace n with {7}
(5 x 7)² + 5 = 35² + 5 = 1225 + 5 = 1230
(9 x 3²) - 7 = (9 x 9) - 7 = 81 - 7 = 74
1230/74 = 16.62 (True)
Replace n with {9}
(5 x 9)² + 5 = 45² + 5 = 2025 + 5 = 2030
(9 x 3²) - 9 = 9 x 9 - 9 = 81 - 9 = 72
2030/72 = 28.194 (False).
b. {5, 7} is your solution set.
hope this helps
