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