It's because you can write "sqrt(96)" as "sqrt(6) x sqrt(16)"

[this is because: sqrt(ab) = sqrt(a) x sqrt(b)]

What do we know sqrt(16) is? 4!

So you end up with -8 + 4sqrt(6), all over 16. 8, 4 and 16 all have a factor of 4, so you can divide them all by 4, therefore simplifying it down to what is shown!