Using the identity (a+b)(a−b)=a2−b2, let's multiply and divide the fraction by 2+√3. You'll notice that, when doing so, at the numerator you'll have (2+√3)2, while at the denominator you'll have (2−√3)(2+√3), which is the formula mentioned at the beginning.
So, (2−√3)(2+√3)=22−(√3)2=4−3=1.
Since the denominator is 1, we can ignore it, and the final answer is thus (2+√3)2