All sides are equal in an equilateral triangle and the perpendicular bisects the side too.
so BC will be bisected and the segments be 5 each.
you can use Pythagoras theorem to find the altitude length, hypotenuse will be one side of the triangle.
[tex] 5^2+h^2=10^2[/tex]
$h^2=100-25=75$
$h=\sqrt{75}=5\sqrt3$
