If you're asking how to add and subtract fractions with different denominators...
Before you add/subtract fractions together, the denominators must be the same. To do this you need to multiply the denominators by each other or multiply it into a number divisible by the original denominators. You then need to do the same to the top (the numerator) as what you did to the bottom (the denominator).
For example, if you had the equation 3/8 + 2/6 you would convert it into 9/24 (*multiplied* by 3) + 8/24 (*multiplied* by 4). You then just need to add the numerators together and keep the denominator the same, so the answer to that equation would be 17/24.
Another example, 4/5 - 2/3 = ?
Convert into: 12/15 (*multiplied* by 3) - 10/15 (*multiplied* by 5) = ? So your answer would be 2/15