4) x² + 5x - 36 = 0
D = 25 + 144 = 169
x1 = (-5 - 13)/2 = -9
x2 = (-5 + 13)/2 = 4
5) x≠-5
2x²-10-4x-20 = 0
2x² - 4x - 30 = 0
x² - 2x - 15 = 0
D = 4 + 60 = 64
x1 = (2-8)/2 = -3
x2 = (2+8)/2 = 5
6) 9x + 12 - 5x²- 20x + 15 = 15
5x² + 11x - 12 = 0
D = 121 + 240 = 361
x1 = (-11 - 19)/10 = -3
x2 = (-11 + 19)/10 = 0,8