I am assuming this is the function although this is not what you wrote.

The denominator cannot be zero, so you must find which values of x would cause the denominator to be zero. Those are the discontinuities in the domain of the function.
If my assumption is correct, then all you need to do is solve these two equations:
x + 21 = 1 x + 3 = 0
x = -21 x = -3
The points of discontinuity are -21 and -3