How can you find the area of a three-dimensional figure?

There are different formulas for each one.

Here are some of them:

Sphere = 4/3 * pi * r^3

Cylinder = pi * r^2 * h

Cone = 1/3 * pi * r^2 * h

Rectangular Prism = 2 (wl + hl + hw)

Cube = 6a^2

The general method would be to take a two or three dimensional integral, but that is most likely not asked in middle school.

For simple volumes, like a cylinder or a cube, you get the total area by summing the areas of all the different sides.