4x + 21 = x²
4x + 21 - x² = 0
-x² + 4x +21 = 0
-x⁶ + 4x + 21 = 0
-1 (x² - 4x - 21) = 0
-1 (x² + 3x - 7x - 21) = 0
-1 ((x² + 3x) + (-7x - 21)) = 0
-1 (x(x + 3) -7 (x + 3)) = 0
-1 (x - 7)(x + 3) = 0
x - 7 = 0
x + 3 = 0
x = -3
x = 7
