3x + 3y = 10

-9 - 9 = -30

Well if your looking for x and y in this situation you won't have number values as your answer, instead you will have a equation.
3x+3y=10
to find x: you have to get x by itself
3x=10-3y
x=(10-3y)/3
or x=(10/3)-y
to find y: you have to get y by itself
3y=10-3x
y=(10-3x)/3
or y=(10/3)-x
I hope this is what you're looking for...