(x−a)(x−b)=x2−x(a+b)+ab
First take L.H.S
(x−a)(x−b)
To multiply two polynomials, we multiply each monomial of one polynomial (with its sign) by each monomial (with its sign) of the other polynomial.
= x.x−x.b−a.x+a.b
= x2−x(a+b)+ab
So, L.H.S = R.H.S
x2−x(a+b)+ab=x2−x(a+b)+ab