Question

What is the quotient a-3/7÷3-a/21

Answer 1

Its -3 I just took the test

Answer 2

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