Note that this is only one of the 2 solutions (the "+" one).
√(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.
So what comes after is also incorrect.
y=3-3√15 is not a solution to the quadratic equation.