20. x⁴ = (2x-15)²

x² = ±(2x-15)

Case 1: x² = 2x - 15 => x² - 2x + 15 = 0. Discriminant = 4 - 4(15) < 0 (no real solutions).

Case 2: x² = -(2x - 15) => x² = -2x + 15 => x² + 2x - 15 = 0.

(x+5)(x-3) = 0

x = -5 or x = 3