2a + 6b = 24, 3a + b = 20

a + 3b = 12, a = 12 - 3b

3(12-3b) + b = 20

36-9b + b = 20

36-8b = 20

8b = 16

b = 2

2a + 12 = 24

2a = 12

a = 6