pls help me with this question from $2016$ exam

Use the coordanate perturbation technique to solve the relativistic differeatual equation

$\frac{d^{2}u}{d{y}^2}=k(1+e{u}^2)-u$

subject to the instal ronditions $u(c,0)=k(1+e)$ and $x^{\text{'}}(e,0)=0$

$25$