Explanation:
1/3 - 1/2
First, balance the denominator by multiply 1/3 with 2, both numerator and denominator
1 x 2
-------
3 x 2
=2
--
6
do The same step with 1/2 but multiply it with 3
1x3
-----
2x3
=3
---
6
now just substract your new fraction
=2/6-3/6
= -1/6