Yep!

Note that this is only one of the 2 solutions (the "+" one).

Yep, someone dropped the 6 by accident.

The 4 was factored out of the square root.

√(4*15) = √4 √15 = 2 √15

At the same time both nominator and denomitor were divided by 2.

As you can see the lost 6 in the denominator magically reappeared here.

I'm afraid his step is incorrect.

So what comes after is also incorrect.

y=3-3√15 is not a solution to the quadratic equation.