4a+6b=10

2a-4b=12

what does 12a = ?

1/2 (4a + 6b = 10)

2a + 3b = 5

2a + 3b = 5

-) 2a - 4b = 12

7b = - 7

b = - 1

Substitute:

2a - 4 (-1) = 12

2a + 4 = 12

2a = 8

a = 4

12a = 12 (4) = 48

This is a little longer way to do it, but I think it's easier to understand