Time needed for photon to reach the surface of a white dwarf [closed]

How long does it take (in terms of years) for energy (a photon) to get from the core to the surface of a white dwarf with effective temperature of 30,000K and average inner temperature of 15MK ? I think the formula we should employ is: $$t = (3R^2) / (lc)$$ where $R$ is the radius of the star, $l$ is the mean free path and $c$ is the speed of the photon. I can't figure out how we can calculate $R$ and $l$ based on the information given in the question.

You can't calculate $R$ and $l$ from the information given. $R$, the radius of the star, has nothing to do with electron scattering through it---i.e. it's an external parameter that you have to know or look up (White Dwarf radius is a good value to remember --- it's basically the same as the Earth!). The mean free path you determine by the properties of the medium through which you are scattering.