I have this question to express it in a specific form
Express $\dfrac{1}{\sqrt{2} + \sqrt{3} + \sqrt{7}}$ in the form $a \sqrt{2} + b \sqrt{3} + c \sqrt{7} + d \sqrt{42}$, for some rationals $a,b,c$ and $d$.
So the solution that I have and am trying to understand is this
So what I don't understand is that why in the world is there a 2sqrt6 here where did it come from?