First, we need to get rid of the parentheses. We do this by dividing by four. Remember that, because multiplication and division distribute, we will need to divide each term by four.
1/4 (20 - 4a) = 6 - a
20/4 - 4a/4 = 6 - a
5 - a = 6 - a
Now, we need to get "a" by itself, so we add a to both sides.
5 - a + a = 6 - a + a
Add like terms.
5 = 6
Because this is not a true statement, there is no solution.