Multiply and simplify(x-3y) (x + 3y)
\((x-3y) (x + 3y)=x^2-9y^2\\\\ (a-b)(a+b)=a^2-b^2\)
\((x-3y)(x+3y) \\ =x^2+3xy-3xy-9y^2 \\ =x^2-9y^2\)
