Question 6919
1) 2(x-y) = 6
2) 2x - 2y = 6    <---- expanding the brackets
3) 2(6) - 2y = 16 <---- substituting in the value of x
4) 12 - 2y = 16
5) -2y = 4        <---- subtracting 12 on both sides
6) 2y = -4        <---- multiply by -1 on both sides
7) y = -2         <---- divide by 2 on both sides

Hope that helps.
Regards,
-Mike