multiply second equation by 2 and rewrite
4a + 6b = 10
4a - 8b = 24
subtract equations
0 + 14b = -14
14b = -14
b = -1
sub back into 4a + 6b = 10 to find the value of a
4a + 6 (-1) = 10
4a - 6 = 10
4a = 16
a = 4
therefore, 12a = 12×4
