Step-by-step explanation:
First remember, a(c-d)=ac-ad
1/3(21-3a)=6-a then
21/3 - 3/3a =6-a
7-a = 6-a
here it is impossible because if we add a to both sides we get 6=7 which is obviously not true. You might have wrote the question wrong, let me know.
