Interpolation Equation

[MNH]

The following is from my online Hydraulics and Hydrology class.

The required elevation for a volume of 0.46 acre-ft. is somewhere between 74 ft. and 75 ft. The
accumulated volumes are therefore between 0.39 acre-ft. and 0.65 acre-ft. Use the interpolation
equation to figure it out.

I never heard of the term interpolation equation. Is it the same as linear regression? I found
y - y1 = [(y2 - y1) / (x2 - x1)] (x - x1) on Wikipedia. I could easily write something for my HP 48G to crunch these numbers, but I figure there's already something on the HP 48G to do the same thing. There always seems to be a semantics issue involved when trying to figure out how to do something like this.

My class learning module gives an example of calculating this using Microsoft Excel, but since I can't read the blurry text in the formula bar, I'm resorting to doing the math on my calculator.