18 min left anyone know the answer?

You have a 30-60-90 triangle.
The short leg is opposite the 30-deg angle. It is y.
The long leg is opposite the 60-deg angle. It is x.
The hypotenuse is opposite the right angle.

The ratio of the lengths of the short leg to the long leg to the hypotenuse is:

1 : sqrt(3) : 2

That means that hypotenuse is twice the length of the short leg.

Your hypotenuse is 10sqrt(3). The short leg is 5 sqrt(3).

The long leg is sqrt(3) times the length of the short leg.
The short leg measures 5sqrt(3), so the long leg measures 
5sqrt(3) * sqrt(3) = 5 * 3 = 15