How do i work out: 3/4-3/5

In order to add or subtract fractions, they must have the
same denominator.  You need to change both of these
fractions to have the same denominator ... without changing
their values, of course.

The smallest common denominator for these fractions is  20 .

     3/4  is the same amount as  15/20 .

     3/5  is the same amount as  12/20 .

Now that they have the same denominator, you can do
the subtraction.