It usually works well to follow the instructions in the problem.
a. Start by making a graph.
b. The curve has decreasing slope, so could be a log or square root function. The square root function best describes the data. A graphing calcluator can help you decide here. (It is also helpful to have a little background in physics, where you learn that a pendulum period is approximately proportional to the square root of its length.)
c. For this, you draw a horizontal line on your graph at time = 4.5 s, and read the string length value. It may be about 5 or 5.1 m.
d. For this, you draw a vertical line on your graph at length = 14 m, and read the period. It may be about 7.5 s.
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Note: you are not being asked to find the function, just characterize its shape. The answers to parts c and d come from reading your graph.