Usually, to add or subtract 2 fractions, they both need to have the same denominator.
5/4 and 3/4 both have the denomimator 4, so we can group them into 1 fraction:
5/4 - 3/4 = (5-3)/4 = 2/4;
Now you need to simplify the fraction. Since 2 is factor of both 2 and 4, you divide both of them by 2:
2/4 = (2/2) / (4/2) = 1/2;
Hope this helps! :)