Answer:
You should multiply the expression by

Step-by-step explanation:
To rationalize any expression, you must multiply it by its conjugate. A conjugate is defined as a similar expression to the original one but with an opposite sign
This means that:
The conjugate of a + b would be a - b
Now, the given expression is

Consider the denominator:
From the above, we can conclude that the conjugate of
is

And, remember that we need to keep the value of the expression unchanged. This means that we must multiply both the numerator and the denominator by the same value
Therefore:
You should multiply the expression by
in order to rationalize the denominator
Hope this helps :)