can someone show me an example in finding the theoretical plate by using tangent line method and the data of a graph?

We can't do graphs here but I googled it and this is what I came up with. The summary shows what you want; just look at each of the sites and see which ones go with the method you want.

https://www.google.com/search?client=firefox-b-1-d&q=example+of+finding+number+of+theoretical+plates+using+the+tangential+method