Answer
$k = 0.299~N/cm$
It is better to use the value obtained from the best-fit line because this value will be more accurate.
Work Step by Step
The first data point is $(0~N, 9.4~cm)$
The last data point is $(6.00~N, 29.5~cm)$
We can find the slope:
$slope = \frac{\Delta L}{\Delta F} = \frac{29.5~cm-9.4~cm}{6.00~N-0~N} = 3.35~cm/N$
We can find $k$:
$k = \frac{1}{slope} = \frac{1}{3.35~cm/N} = 0.299~N/cm$
It is better to use the value obtained from the best-fit line because this value will be more accurate.
