You can't simplify 5/6 since while 6 would divide in half to be 3 and match the other denominator, 5 wouldn't divide in half evenly. Instead, multiply the top and bottom number of the other fraction by two:
5/6 - 4/6
This way they share equal denominators. When subtracting fractions with matching denominators, the denominator doesn't change in the answer. Just subtract the numerators:
