12x + 6 - 5x = 20 |-6
12x - 5x = 14
7x = 14 |:7
x = 2
SPRAWDZENIE
12 * 2 + 6 - 5 * 2 = 20
24 + 6 - 10 = 20
30 - 10 = 20
20 = 20