The equation relating flux density (\$B\$), core permeability (\$\mu\$) and magnetic field strength (\$H\$) is: -
$$B = \mu H$$
Note that \$H\$ is the driving force of magnetism i.e. current, number of turns and distance of the flux path. So, you need to know the cores magnetic permeability if you are to find flux density. Given that your question doesn't cover this option, you cannot calculate flux density.