Решить через дискриминант.
20x^2-20x-400=0

a = 20; b = -20; c = -400D = b^2-4ac = (-20)^2 - 4 * 20 * (-400) = 400 + 32000 = 32400.x¹,x² = (-b+/-√D)/2ax¹ = (20 - 180)/40 = -160/40 = -4x² = ( 20 + 180 )/40 = 200/40 = 5.