First, the parentheses are resolved by multiplying '7' through each term inside the parentheses.  The final simplification is obtained by matching up numeric values and terms with 'm'.  Clicking on a particular solution step is the fastest way to review the logic which is employed and sometimes clicking on the 'Options' tab and choosing 'a few steps' can provide an easier presentation of the simplification... sometimes 'a lot of steps' is helpful in order to nail down exactly what is happening at each solution step.