I struggle to simplify the following equation: (see answer)

3l²mn + nl²m - 5mn²l + l²nm + 2n²ml - mn²
The first step is to ask yourself if there is anything that could help you simplify the equation, any recurring term or common factor. Here, we note that mn is a common factor so let us try to factorise the equation using mn as the main factor, as follows:
3l²mn + nl²m - 5mn²l + l²nm + 2n²ml - mn² = mn (3l² + l²-5nl + l²+ 2nl- n) = mn( 5l² -3nl - n)
This is the most simplified version of this equation, thus we have rightly answered the question.
Note: When factorising, be careful about quadratic terms to avoid silly mistakes, rigour is needed!