Check the previous step.
It says (a-b)^2.
It is the same as saying: (a+b)^2-4ab
Because if you expand it, you will be getting (a-b)^2 again. Hence, there's no disturbance in the equation.
It's done to get the value of (a-b).
Hope you understand. If you have any doubt, please reply and I'll try to help.
Hope that helps.
Cheers!