How do you multiply #[a^(1/2) + 6^(1/2)] [a^(1/2) - 6^(1/2)]#?
You could multiply out out using FOIL, or whatever idea you've been taught for multiplying binomials.
It's quicker to recognize that this product is of the form:
(The product of a sum and difference equals the difference of the squares.)
Now realize that