I am unable to understand the context of the doubt. You have asked why the step exists. Which means you are referring to a solution that you have already seen somewhere. Right? Can you pls show the screenshot of that solution so that I can help out.
I think the simplest answer would be this though.
These two equations have already been given and understood
y+x = 3a --> y+ (x-3a) = 0
y-x = -3a --> y- (x-3a) = 0
Multiplying both sides of the given equations lets you arrive at the step in question.
Does this help? If not, pls show the solution you are referring to so that we can proceed.