I wish you were using parentheses here. I must assume (without knowing for sure) that y ou mean
a - 3/7
----------
3 - a/21
The LCD here is 21, since 7 divides into 21 without a remainder.
Thus, multiply numerator and denominator by 21:
21a - 9
-----------
63 - a
You could leave this result as is, or you could factor out 3:
7a-3
3 ---------
63-a
