If there are two values places next to each other (E.g. xy), then the values are multiplied together. In this example, "3n - 2m" is one value as it is in the brackets. The second value is "2".
To simplify, you must open up the brackets. So multiply what is inside the brackets by what is outside. (2 x 3n) - (2 x 2m) = 6n - 4m