Captionless Image please help me

Let's start with the basic idea that we can multiply a number by 1 without changing its value, and that we can express 1 as a fraction with the same numerator and denominator:

In cases like these (i.e. where the denominator is a square root) we can multiply both numerator and denominator of the fraction by the square root itself:

But the square of a square root is the agument of the root, i.e. , so we have

And we rationalized the denominator, as asked.