Question 145067
The key is that an "equilateral triangle" implies that ALL three sides are of equal lengths.  Once you know that (if you draw a diagram), you can see that the "altitude" will bisect (divide it in half) one side of the triangle (at exactly 90 degrees).  Now you can apply Pythagorean's theorem:

Let x = length of a side of the equilateral triangle
17.32^2 + (x/2)^2 = x^2

Solving for 'x' above will yield your answer.