Solve the system by substitution, if possible.

y=9x-21   3x-y=3

Note that we can substitute the first equation into the second for y......and we have....

3x - [ 9x - 21]  = 3      simplify

3x - 9x + 21  = 3

-6x + 21  = 3      subtract 21 from each side

-6x  = -18            divide both sides by -6

x = 3     and to find y....we can use   y = 9(3) - 21  = 27 - 21   = 6

So    {x, y}  = {3 , 6}