#### Answer

The length of the altitude is 6 cm.

#### Work Step by Step

Remember that the formula for the area of a triangle is A = $\frac{base \times \ height }{2}$.
Let the 9 centimeters side be the base of the triangle. The altitude to this side will be the height of the triangle.
We know the area and we know the base, so we plug these values into the equation to obtain the height.
So: 27 = $\frac{9\ \times\ height }{2}$
54 = 9 $\times$ height
6 = height.
The length of the altitude is 6 cm.