The length of the altitude is 9.5 cm.
The altitude bisects the base of the equilateral triangle (this is because the two sides emerging from the vertex are equal). This gives us a right triangle with leg 5.5 and hypotenuse 11. We use the Pythagorean theorem:
5.5² + h² = 11²
30.25 + h² = 121
Subtract 30.25 from both sides:
30.25 + h² - 30.25 = 121 - 30.25
h² = 90.75
Take the square root of both sides:
√h² = √90.75
h = 9.5
