Actually, the altitude will divide the equilateral triangle into two 30-60-90 congruent right triangles.
You know in a 30-60-90 configuration, the altitude is equal to the short side (x) times the square root of 3.
The hypotenue is the length of the side of the equilateral triangle and is two times the short side.
Thus, the side of the equilateral triangle is
If the perimeter of an equilateral triangle is 45, then each side would be 15. This would mean that in your triangular configuration, 15 is the hypotenuse. The short side would be 7.5 (half the hypotenue) and the long side (altitude) would be