The answer is infinitely many solutions.
x - y = 11
-x + y = -11
_____
Express the first equation in the term of x and substitute it into the second equation:
x = y + 11
-x + y = -11
_______
-(y + 11) + y = -11
-y - 11 + y = -11
-y + y = 11 - 11
0 = 0
So, the system has infinitely many solutions.