you need to first make denominator in the fractions you have equal, then simply subtract numenators.
example:
2/3 - 1/2
first step is to make denominators equal.
for this example, the denominator will be equal to 6 which is 2x3
so, 2/3 = 4/6 and 1/2 = 3/6
so, the expression now becomes:
4/6 - 3/6
simply subtract numenators and the denominator will be 6 as well.
4/6 - 3/6 = 1/6
another example:
3/4 - 1/2
here the denominator can be equal to 4 in all terms
1/2 = 2/4
so, the new expression is now:
3/4 - 2/4 = 1/4