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