This is the formula for volume of a cylinder. The volume formula builds off the area formula. Where area is more 2-dimensional, volume is 3-dimensional. You can think of a flat sheet of paper for area (2-dimensional) and a solid box for volume (3-dimensional).
Usually your teacher would give the formulas when starting this section of math. From there, either keep a list of needed formulas in your math folder for easy reference, or try to memorize the formulas for the current section of math you are working on. Some teachers allow formula reference sheets and some do not.
