x+2y=7
3x-2y=-3

you elimin ate one of the variables, e.g. in this case id go with getting rid of the y
take the first line as equation 1 and the second line as 2
(1)= (x+2y=7)
(2)= (3x-2y=-3)
add them to geather
i.e (1)+(2) this makes the y's cancel each other out
so that you get

4x=4

then its easy :)