1/4 (20-4a) = 6-a

solve for a

1/4 (20-4a) = 6-a (distribute 1/4)

5-a=6-a (subtract 5 to both sides)

-a=1-a (add "a" to both sides)

0=1

When this happens where the equation is not true, it means that a=no solution because there would be no "a" value that you could plug in to make the equation true.