Solution
= Solution
The Jacobian is $a^3\det(I+\partial\psi/\partial q)\simeq a^3(1+\nabla\cdot\psi)$, so mass conservation gives $\delta_M=-\nabla\cdot\psi$. Taking the divergence of (i) and using Poisson yields $\delta_M''+\mathcal H\delta_M'-(4\pi G/c^2)\bar\rho_Ma^2\delta_M=0$.