you multiply everything in the parenthesis by the number outside.
In this case its 2 and -1. the first part is 2a-8 the second part is 4a -2a+16a
so it looks like this 2a-8=4a-2a+16a.
combine like terms and get 2a-8=18a.
Subtract the other 2a from the other side and you have 8=16a
divide each side by 16 and you have 8/16 which is one half.
