You want to simplify the following expression:

'12z+5-15z-7'

First we group the like terms and then we perform the operations:

12z+5-15z-7 = (12z - 15z) + (5 - 7) = - 3z - 2 

You can obtain an explanation for any step by clicking on it. Please note that you cannot click on the very first expression, which is the original problem. Quite often, several transformations will be performed within the same step. Use the explanation navigation buttons to step through all of them: