Method 1.
Use the triangle /a picture/
[tex]\dfrac{m\sqrt3}{2}5\sqrt3\ \ \ \ |\cdot2\\\\m\sqrt3=10\sqrt3\ \ \ \ |:\sqrt3\\\\m=10[/tex]
Method 2.
Use the trigonemteric function:
[tex]\cos30^o=\dfrac{5\sqrt3}{m}\ \ \ \ /\cos30^o=\dfrac{\sqrt3}{2}/\\\\\dfrac{\sqrt3}{2}=\dfrac{5\sqrt3}{m}\ \ \ |cross\ multiply\\\\m\sqrt3=2\cdot5\sqrt3\ \ \ \ |:\sqrt3\\\\m=10[/tex]
