The graph shows the relationship between the volume of a rectangular prism and the volume of a square pyramid with an identical base and height.

A coordinate plane showing Prism versus Pyramid. The x-axis shows Pyramid Volume in cubic units and the y-axis shows Prism Volume in cubic units. The line starts at (0, 0) and passes through (1, 3), (2, 6) and (3, 9).

What is the slope of the line?

−3

3