21)
x^{2} - 3x + \sqrt{6-x} = \sqrt{6-x} + 40
x^{2} - 3x + \sqrt{6-x} - \sqrt{6-x} - 40 = 0
x^{2} - 3x - 40 = 0
D = 3^{2} - 4 * 1 * -40 = 9 + 160 = 169;
\sqrt{D} = 13;
x1 = (3 + 13) / 2 = 8;
x2 = (3 - 13) / 2 = -5;
22)
x - 1 = (x + 7) * (2/3)
x - x*2/3 = 7*2/3 +1
x = (7*2/3 + 1) * 3
x = 14 + 3 = 17км/ч