Well, first of all, 10³⁰ kg is its mass, not its weight.
-- Density of anything is (its mass) divided by (its volume).
-- You know the white dwarf's mass.
-- The volume of any sphere is (4/3) (pi) (radius)³
-- The radius is 1/2 of the diameter.
-- Now you have the ball. Run with it.
I did it quickly and got (1.91 x 10¹²) kilogram/meter³ .
That may be wrong. You have to check it. And even
if it's correct, you probably want to express density in
units of gram/cm³ the way everybody always does.