With fractions, you are allowed to multiply or divide the top and bottom of the fraction by some number, as long as you multiply (or divide) everything on the top and everything on the bottom by that number.
Adding and subtracting fractions
To add two fractions, the bottom (denominator) of the two fractions must be the same.
If the denominators are not the same, multiply or divide the top and bottom of one (or both!) of the fractions by a whole number to make the denominator of each fraction the same.
The same is true when subtracting fractions.
This is simple, just multiply the two numerators (top bits) together, and the two denominators (bottom bits) together:
If A, B, C and D are any numbers,
These rules work even when the fractions involve algebra.
A note on cancelling
You can divide or multiply the top and bottom of any fraction by any number, as long as you do it to both the top and bottom. However, when there is more than one term on the top and/or bottom, to cancel you must divide every term in the top and bottom by that number.
Here there is only one term in the numerator (top) and denominator (bottom) of the fraction, so you can divide top and bottom by 2.
Simplify the expression:
In questions such as this, it is often useful to factorise first to get: