How do I solve
$$af(z)+c \int_{z}^{0} \int_{-h}^{z} f(t)dt^{2}=d $$
while $-h<z<0$ and $a,c,d$ are real constants. I tried to rewrite the equation as $$a{y}''+cy=d $$ where, $y(z)=\int_{z}^{0} \int_{-h}^{z} f(t)dt^{2}$. However, I couldn't find the solution.