Simplify $(6\sqrt{x} + 3\sqrt{y})\cdot(6\sqrt{x} - 3\sqrt{y})$.

Question

This expression $(6\sqrt{x} + 3\sqrt{y})\cdot(6\sqrt{x} - 3\sqrt{y})$ equals: ?

Simplify the expression.

I'm tried multiplying but I'm not getting the right answer.

Answer 1

Just use $a^2 - b^2 = (a+b)(a-b)$ and get the answer as $36x-9y$

Answer 2

Hint: Use $$(a-b)(a+b)=a^2-b^2.$$