I'll try to get you pointed in the right direction and assume that you know some algebra and can finish the problem on your own.
You can find an expression for x by eliminating y from the two equations. To do that, multiply each equation by an appropriate constant so that the coefficients of the y terms in the two equations are the same. Then subtracting one equation from the other will eliminate y, giving you an equation you can solve for x.
ax + by = m
cx + dy = n
** This line modified in response to reader's question **
Multiply the first equation by d and the second by b; then subtracting the second equation from the first eliminates y, allowing you to solve for x:
adx + bdy = dm
bcx + bdy = bn
---------------
(ad-bc)x = dm-bn
x = (dm-bn)/(ad-bc)
You COULD plug that expression for x into one of the original equations to find the expression for y; but that would be ugly. It's easier to use the same procedure as above for solving for y.
ax + by = m
cx + dy = n
Multiply the first equation by c and the second by a to make the coefficients of x the same. Then, similar to above, subtract one equation from the other to eliminate x, allowing you to solve for y.
(