To divide (a-3/7) by (3-a/21), we need to multiply the numerator by the reciprocal of the denominator.
Reciprocal of (3-a/21) is (21/(3-a))
So,
(a-3/7) / (3-a/21) = (a-3/7) * (21/(3-a))
Now, we can simplify by canceling out common factors between the numerator and denominator.
(a-3/7) * (21/(3-a)) = (3-a)(a-3)/(7*21)
= -(a^2 - 6a + 9)/147
= -(a-3)^2/147
Therefore, the quotient is -(a-3)^2/147.