9*(х-5)+4(6-х)=3+3*(х+20)-2х

9 * (х - 5) + 4 * (6 - х) = 3 + 3 * (х + 20) - 2х
9х - 45 + 24 - 4х = 3 + 3х + 60 - 2х
5х - 21 = 63 + х
5х - х = 63 + 21
4х = 84
х = 84 : 4
х = 21

проверка:
9 * (21 - 5) + 4 * (6 - 21) = 3 + 3 * (21 + 20) - 2 * 21
9 * 16 + 4 * (-15) = 3 + 3 * 41 - 2 * 21
144 - 60 = 3 + 123 - 42
84 = 84