Geometry help

Hey Guest! 

This is a basic trigonometrey relationship. 

We use cosine in this case, where cosine is adjacent over hypotenuse. 

$\cos{30º}=\frac{x}{10}$

$\frac{\sqrt3}{2}=\frac{x}{10}$

$x=\boxed{5\sqrt3}$

I hope this helped,

Gavin