For part b use the end point, which is where the vertical region is. For this graph this is 25cm3. We know the mole ratio of HNO3:NaOH, so find the concentration that way.
For part c this isn't actually a buffer because although there is sodium nitrate in the solution, we also have NaOH which is a strong alkali and it doesn't not form a buffer. I do edexcel and we ignore the salt, but I'm not sure if you approximate it the same way with OCR. If they do, think of what is left in the solution when 50cm3 is added and calculate the concentration ignoring the salt