7(a-3)=3(4a+21)

Solution for 7(a-3)=3(4a+21) equation:

7(a-3)=3(4a+21)

We simplify the equation to the form, which is simple to understand 
7(a-3)=3(4a+21)

Reorder the terms in parentheses
+(+7a-21)=3*(4a+21)

Remove unnecessary parentheses
+7a-21=+3+*(+4a+21+)

Reorder the terms in parentheses
+7a-21=+(+12a+63)

Remove unnecessary parentheses
+7a-21=+12a+63

We move all terms containing a to the left and all other terms to the right. 
+7a-12a=+63+21

We simplify left and right side of the equation. 
-5a=+84

We divide both sides of the equation by -5 to get a. 
a=-16.8