Understanding Cosmological Perturbation Equations

I am writing to numerically solve the following coupled first order partial differential equations. First I am trying to rewrite them in a more favorable way. In ther equations below $Theta_{r,0}$ and $Theta_{r,1}$ represent photon perturbations from scattering. $Phi$ represents a perturbation due to the gravitational potential, $delta_c$ is perturbation due to cold dark matter and $u_c$ is speed of the dark matter. Here a prime represents a derivative with respect to conformal time. Each is a function of $k$ and $eta$, which is the conformal time. I was able to rewrite the first four equations, but I am having trouble with an Einstein equation (see ch 8 in Dodelson’s Modern Cosmology):

$k^2Phi + 3frac{a’}{a}bigg(Phi’ + frac{a’}{a}Phibigg) = 4pi Ga^2big[rho_c delta_c + 4rho_rTheta_{r,0}big]$.

I was able to rewrite $frac{a’}{a}$ as $frac{1}{eta}$, but I still have the $a^2$ on the RHS. Is there any way I an express $a$ in terms of conformal time? Basically, I want to have all variables as either $k$, $eta$ because that is what the perturbations, etc are a function of.