simple radical form with a rational denominator

Question

express the reciprocal 3-√7 in simplest radical form with a radical denominator

Answer 1

I am not sure what is wanted here, but...

1/(3-√7) = (3+√7)/(9-7) = (1/2)(3+√7)

There is a sort of mnemonic I think worth remembering:

The sum of 2 numbers times the difference of two numbers is the difference of their squares.