substitution problem?

6x-y= -4 ------>(1)
2x+2y= 15 ----------->(2)

multiply eqn(2) with -3

6x-y= -4
-6x-6y= -45

solving above eqns

-7y=-49
y=7

sub. y value in
6x-y= -4
6x-7= -4
6x= -4+7
6x= 3

x=1/2
y=7

this can be verified by sub. in given eqns

12x - 2y = - 8
2x + 2y = 15--------ADD

14x = 7
x = 1/2

1 + 2y = 15
y = 7

x = 1/2 , y = 7

6x - y = -4
2x + 2y = 15

6x - y = 4
y = 6x - 4

2x + 2y = 15
2x + 2(6x - 4) = 15
2x + 12x - 8 = 15
14x = 15 + 8
x = 23/14

6x - y = -4
6(23/14) - y = -4
69/7 - y = -4
y = 69/7 - (-4)
y = 69/7 + 28/7
y = 97/7

∴ x = 23/14 , y = 97/7

6x-y=-4
6x=y-4
x=(y-4)/6

2x+2y=15
(y-4)/3+6y/3=15
7y-4=45
7y=49
y=9

6x-y=-4
6x-9=-4
6x=5
x=(5/6)

so y=9 and x=5/6

The same sort of way you'd solve it for the last question you posted:
http://answers.yahoo.com/question/index?qid=20090306234412AA1nNsZ&r=w#QJB8WjW4UDL.jfZGj9PfMGYs9kShWxKlKcD3NSPuzd78k3.WpajY

I'd start by solving the first equation for y.