Please help with attached question :

As part of a clinical trial, a study is conducted to measure the survival rates of patients who have undergone a certain surgical procedure. Each patient was observed from their date of operation until their date of exit. The results are reported in the table below. Patient ID Date of operation Date of exit Reason for exit I July 2018 1 July 2019 Censored 1 June 2019 1 January 2020 Censored 1 April 2019 1 January 2020 Censored 1 November 2018 1 January 2019 Death 1 March 2019 1 November 2019 Death 1 April 2019 1 October 2019 Death 1 February 2019 1 January 2020 Censored 8 1 August 2019 1 January 2020 Censored 1 May 2019 1 November 2019 Death 10 1 January 2019 1 January 2020 Censored Table 2: Observation over patient's entry and exit time and the reason of exit. Assume that the censoring mechanism was non-informative with regard to the survival of any individual patient. (a) Calculate the Nelson-Aalen estimate of the cumulative hazard functions. A(). where / is the time (in months) since the operation. (b) Calculate an estimate of the Kaplan-Meier survival function for patients who have had this operation. (c) Use estimated variance of the survival probability $ xx(?) given by Var(S KM(D) = SKM(D) > to get estimated variance for survival probability of a patient at least 10 months after having the operation