Use Graphical Analysis to make a graph of the period versus the length. The shape of the graph is different from any other you have done. Choose "Automatic Curve Fit" from the Analyze menu. Click on "Power." The resulting graph is a half-parabola around the x-axis. It means that the variable on the y-axis is directly related to the square root of the variable on the x-axis. Express this relationship using the variables you analyzed instead of the generic "y" and "x."