Solve 9x-3y=9, x+3y=11

9x-3y=9, 
x+3y=11 

adding both equations: 

(9x-3y)+(x+3y)=9+11 
9x +x = 20 
10x=20 
x=2 


now using the 2nd equation 

x+3y+11 
2+3y=11 
3y=9 
y=3 

solution: (2,3)