what can * by -168 and adds to get 21.
I assume your question is
which two numbers have a product of -168 and a sum of 21?
Well, check the factors of 168:
2 84
3 56
4 42
8 21
6 28
12 14
Now, which two differ by 21? None. So, your numbers are not rational. I suspect a typo.
Algebraically,
xy = -168
x+y = 21
x + (-168/x) = 21
x^2 - 21x - 168 = 0
x = (21±√1113)/2