The rule is: Before you can add or subtract fractions, they must have
the same denominator.
For this little problem, don't worry about the signs yet. Just make both
fractions have the same denominator. I suggest you change the first one
to something over 250.
When your fractions have the same denominator, you get the sum or difference
like this:
-- The numerator of the answer is the sum or difference of the two numerators.
-- The denominator of the answer is the same as the denominator of the two fractions.