Answer
$15.1\;\rm cm$
Work Step by Step
We need to use the thin lenses equation of
$$\dfrac{1}{s}+\dfrac{1}{s'}=\dfrac{1}{f}$$
We need to find the focal length by using the best-fit line.
So,
$$\dfrac{1}{s}=-\dfrac{1}{s'}+\dfrac{1}{f}$$
we can see that this formula is a straight-line formula $y=mx+b$ where $y=s^{-1}$,
$x={s'}^{-1}$,
$m=\rm{slope}=-1$,
and
$b=f^{-1}$.
So after we plot the given data, the $y$-intercept will be equal to
$$b=f^{-1}$$
So,
$$f=\dfrac{1}{b}\tag 1$$
Noting that $s$ is the Bulb to lens distance, $s'$ is the image [screen] to lens distance.
From the graph below, we can see that
$$b=0.06626$$
Hence,
$$f=0.06626^{-1}=\color{red}{\bf 15.1}\;\rm cm$$
