Source: US Department of Energy

c). Estimate the natural gas consumption in 1982.

Solution:
The approximate natural gas consumption for 1982 is given by f(1982), where the modeling function has been found to be:

Press 2nd, then TRACE[CALC]. Select 1:value by pressing ENTER. Enter 1982 as a value for x=, then press ENTER.

The corresponding y value appears at the bottom right of the screen. That value is approximately 20.1. Thus, the natural gas consumption for 1982 is approximately 20.1 quadrillion BTU.