first, we have to make one of the x's have the same coefficient so they can cancel each other out (or eliminate)
3(x+y) = 3(2)
you're going to have to do it to both sides to keep it balanced
3x+3y=6
now line them up and add them
3x+3y= 6
+ (-3x+4y=15)
since the 3x and -3x add up to 0x and cancels each other, you're left with
7y = 21
divide both sides by 7
y=3
plug in the y into one of the original equations
x+y=2
x + 3 = 2
subtract 3 from both sides
x = -1
x = -1 , y = 3