Rating:

tldr;
- since $Q = kP$ and $R = (k+1)P$, then $R - Q = P$
- use the addition formula and the elliptic curve equation and $Q_x R_x = N$ to get five equations in the four unknowns $R_x, R_y, Q_x, Q_y$
- solve the system of multivariate polynomials (e.g. with resultants) to recover $R_x$ and $Q_x$

Original writeup (https://jsur.in/posts/2021-03-07-zer0pts-ctf-2021-crypto-writeups#not-mordell-primes).