for finding difference or sum of unlike fractions ( fractions with different denominators) then we need to make their denominators same ...so for that we take their denominators LCM ...
7/4 - 1/8 (LCM of 4 and 8 is 8)
=> 7/4 X 2 - 1/8
=> 14/8 - 1/8
= 13/8 ..
Hope it helps !!!