The third step towards the solution is understanding that one can apply an additional condition to the statement of the problem, as long as it does not contradict the original conditions. Choose such an additional condition which makes it easy to construct the focal plane of the lens.
Please submit the solution to this problem via e-mail to physcs.cup@gmail.com. The next hint for the Problem 4 will be published at 13:00 GMT, 13th March 2022, together with the next updates of the intermediate results.