x^2-6x(y-z)+9(y-z)^2
=x^2-6x(y-z)+[3(y-z)]^2
=(x-3y+3z)^2
(a+b)^2-4(a+b)c+4c^2
=(a+b)^2-4(a+b)c+(2c)^2
=(a+b-2c)^2