The table below shows measurements of magnification versus object distance for a lens. Determine a quantity that, when you plot object distance against it, should give a straight line. Make your plot, establish a best-fit line, and use your line to find the focal length of the lens.
$$
\begin{array}{|l|c|c|c|c|c|}
\hline \text { Object distance, } s(\mathrm{~cm}) & 10.1 & 29.2 & 51.6 & 78.3 & 98.9 \\
\hline \text { Magnification, } M & 1.31 & 4.77 & -4.38 & -1.27 & -0.724 \\
\hline
\end{array}
$$