You can put this solution on YOUR website! An equilateral triangle has all sides equal.
If the perimeter = 30 inches, then the sides
all equal 10 inches.
An altitude from the top apex of the triangle
will cut the base of the triangle into 2 each
equalling 5 inches.
You then have a right angled triangle with a hypotenuse
= 10 inches and a side = 5 inches
The altitude^2 = 10^2 - 5^2
Altitude = square root of 75
Altitude = 8.7 inches (1 decimal place)
Hope this helps. :-)