Hi Nidhi,
You will need to replace the words Start and End with an empty string in order to get the resultant string between them. Then you can Trim it if you also need to remove the spaces in them.
So you will need to use a combination of the Replace and the Trim functions. You can use a single 'Multi Calc' stage instead of several Calc stages.
PS: Since you asked "which function to use" it looks like you want to practice it on your own so respecting that I haven't pasted the direct function line here . But please feel free to post back in case of any issues in doing so.