The factors of -3 are 1 and -3 or -1 and 3, no summation of which results in +3, hence the quadratic is not factorable over the integers. Therefore, you must either complete the square or use the quadratic formula. Since the quadratic formula is nothing more than the general case of completing the square, let's use it:
For
So for
.
Since there are no perfect square factors of 21, this is simplest form.