How to find the fundamental frequency and present harmonics from a graph?
First of all you should look up the Fourier Transformation/Series.

But you can do some Assumptions based on the time function:

First off ask yourself does the Signal have a DC Offset. In our case yes.
So we would have a frequency part at f=0 in the frequency domain of A/2.

Now we look at the fundamental frequency of the Signal. As the period is 20ms it would be 50 Hz. This is our fundamental frequency.

Now we can look up if our time function is point symmetric or axis symmetric. To get information if the resulting frequencies are even or odd.