How do you find the length of the altitude when they only give you the premiter?

Ex: preimeter of the equilateral triangle is 36, what's the altitude?

Do you mean Perimeter?
If I'm not mistaken, the altitude is the height.

See if this site helps.
Mathwords: Altitude of a Triangle

Just in case you don't know, an equilateral has its three sides of the same length. That means that each side is 6 from your example.

Now, draw the triangle on a piece of paper. The base is 6 and the altitude is the length from the base to the highest point of the triangle. A vertical line can be drawn to represent the altitude. This line starts from the base up to the highest point.

Now, you need to use the Pythagoras' Theorem to find that altitude, using one slant side and the base. You'll notice that you have to take half the length of the base to find the altitude.

If you need something, ask because I cannot know what you don't know. :)

Draw unknown008's diagram of an equilateral triangle with 12 on every side and draw a line from the top to the bottom (altitude).
You should realize that this is 2 right triangles that are similar and congruent.
Now since the line is vertical and starts at the top of an equilateral triangle it divide the bottom in to 6 units each
Now simply use the Pythagoream Theorum
a^2+b^2*=c^2
*b is the altitude and a is a 6 and obviously the diagonal is 12 so
6^2+b^2=12^2
36+b^2=144
b^2=108
b=sqrt of 108 which is around 10.4