First of all, you can simplify the 4 at the numerator and the 14 at the denominator (they're both multiple of 2):
Now, rationalize a denominator means that you have to get rid of the square root, in order to have an integer denominator.
To do so, remember that you can always multiply any number by 1 without changing its value, and you can always think of 1 as a fraction where numerator and denominator are equal: