Simplify each of the following expressions.
Your answer should be a polynomial, that is, a sum of terms which are each a coeficient times powers of the variables.

a) $(-2 x^2+6 xy-2 y^2)+(2 x^2-9 xy)=$

b) $(-4 x^2+6 xy-2 y^2)\times (6 x^2-9 xy)=$