Justification for removing negatives on this algebra symbol manipulation question:

Hi folks! Certainly glad this forum exists

I'm following on how to do this till the very last step where -x1 is on the left and -x2 is on the right. I don't understand why one can simply remove both negatives or why they would negate each other on opposite sides of the equation?

Here's the the question and solution:

x1 & y1 and x2 & y2 are two points on a non-vertical line. X1 does not equal x2. The slope (M) of the line is:

Multiply on both sides, and divide M on both sides to begin isolating x1.Then, by subtracting http://www.aleks.com/alekscgi/x/math...D1%3F%2Cpva%3D from both sides of the equation, we get