It is all the same if you write "x^2 - 2x + 1" or "x^2 + 2*x(-1) + (-1)^2".

We want to use the binomial formula, so we write the equation in particular way:

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, there might be several transformations will be performed within the same step.