In about 5 billion years, at the end of its lifetime, our sun will end up as a white dwarf, having about the same mass as it does now, but reduced to about 15,000 km in diameter. what will be its density at that stage?

It is fine to use the equation given by Plitter, since we are told that the mass is about the same as it is now, and I seriously doubt the original question wants the student to go into relativistic effects, electron degeneracy pressure and magnetic effects that govern a real white dwarf star.
There is no need to make it unnecessarily complicated, when the question is set at high school level.  The question asks, given a particular radius, and a given mass, what will the density be (which in this case will be the average density).   To answer the question, one needs to know the mass of the sun (which is about 2×1030 Kg.  One needs to convert the diameter to a radius, and then calculate the spherical volume of the white dwarf.  Then one can use the formula given above, namely density=mass/volume