Hi Mellie :)
$$\\In $\triangle ABC$, $AB = 5$ cm, $BC = 10$ cm, and the altitude drawn to $\overline{AB}$ is 8 cm. $\\$What is the number of centimeters in the length of the altitude to $\overline{BC}$?$$
Let h=altitude to BC
When I tried to draw the pic I realized that the altitude to AB had to lie outside the triangle.
That is, angle CAB is an obtuse angle.

Area of triangle ABC = 1/2 * 5*8 = 20cm^2 (error fixed - thanks Chris)
Area of triangle also = 1/2 * BC * altitude to BC = 1/2 * 10* h = 5h cm^2
so 5h=20
h=4
The altitude to BC is 4 cm