First, fill the first column with the length of the string of the different pendulums:
Fill the next three columns of each row according to the time obtained using the simulator:
Take te average of the period for each row in order to obtain the data for the column Average Period:
Then, the fifth column is:
Square the values of the fifth column to fill the sixth column:
Then, the fourth column is:
Use a graphing facility to find the best fit line for each data set (average period vs length and square of average period vs length):