Question 141090
The line you have runs from the vertex of one triangle to the opposite side correct?

Since the triangle is regular (equilateral) all 3 sides are the same length. The line from the vertex to the center of the opposite side creates a right triangle with sides 10, x, x/2 . Where x is the hypotenuse.

So use the Pythagorean theorem to solve for x

{{{x^2 = (x/2)^2 + 10^2}}}
{{{x^2 = x^2/4 + 100}}}
{{{(3x^2)/4 = 100}}}
{{{x^2 = 400/3}}}
{{{x = 11.55 }}}

Area = b*h = 11.55*10 = 115.5
Perimeter = 3*b = 34.65

Is that what you get ?