Background
I am trying to determine the value of g based on a simple pendulum. The equation, for a small angle approximation works out to L=[G/((2pi)^2)] P^2. Where L is the length of the Pendulum in meters, G is Gravity and P is the period of the pendulum. I have graphed a scatter plot with the x axis as L and the y axis as P.

I have 3 different series for different lengths of pendulum.
When I try to find a line of best fit I get something like ax^2 + bx + c and this trendline is only for one series.

Question
2 Parts - Is there any way to get the trendline to be the line of best fit for all my different series ( different lengths)
- Is there any way to make my quadratic regression approximate only in terms of ax^2.

Thanks
Physics Student