Hello!
Subtract the amount of paint he used from the amount of paint he has:
5 1/2 - 3/4 = 4.75
4.75 as a fraction is: 4 3/4 (As a mixed number) or 19/4 (As an improper fraction).
So, Wyatt used 4 3/4 (or 19/4) of the 5 1/2 gallons of paint he had! :)
And that's your answer! ^ I really hope that was helpful!