Question: how to subtract large fractions with different denominators?
Answer: Cross-multiply the two fractions and subtract the second number from the first to get the numerator of the answer. For example, suppose you want to subtract 6/7 – 2/5. ... 
Multiply the two denominators together to get the denominator of the answer. 7 5 = 35.
Putting the numerator over the denominator gives you your answer.