Note that the polynomial $ b^2+a^2-2ab $ is a perfect square trinomial, so we will use the following formula.
$$ A^2 - 2AB + B^2 = (A - B)^2 $$In this example we have $ \color{blue}{ A = b } $ and $ \color{red}{ B = a } $ so,
$$ b^2+a^2-2ab = ( \color{blue}{ b } - \color{red}{ a } )^2 $$