Algebra

   3x  −y = 6

   6x + y = 21                add these equations

9x    =    27               divide both sides by 9

x  =   3

Using either equation to find y

3(3) - y  = 6

9 -  y  =    6         add y to bothsides, subtract 6 from both sides

3   =   y

So  (x, y)   = (3, 3)