We have two fractions and are being asked to subtract them.
7/8 and 1/3 don't have the same denominator, so to do as so, we need to find the LCM of both denominators, multiply them to that, and do the same for the numerators.
So what is the LCM (Least Common Multiple) of 8 and 3?
This argument is no valid although the first part of the statement is valid. If a number is larger than 6 than its square must be larger than 36. But if the number is smaller than 6, its square does not need to be smaller than 36. For example...