An expression of the form $a^2-2ab+b^2$ can be factored to a perfect square. $$a^2-2ab+b^2=(a-b{)}^2$$ An example is the expression $x^2-10x+25$ which can be written as $x^2-2\cdot x\cdot 5+5^2,$ and thus, be factored to the square of the binomial $x-5.$ $$x^2-2\cdot x\cdot 5+5^2=(x-5{)}^2$$