Hi adambinch, welcome to PF. You need to find out how many atoms there are in the simple cubic unit cell. Then you can set up a calculation like
[tex]\left(\frac{\mathrm{atoms}}{\mathrm{cell}}\right)\left(\frac{\mathrm{weight}}{\mathrm{mole}}\right)\left(\frac{\mathrm{mole}}{\mathrm{atoms}}\right)\left(\frac{\mathrm{volume}}{\mathrm{weight}}\right)=\left(\frac{\mathrm{volume}}{\mathrm{cell}}\right)[/tex]
and the volume per unit cell should lead you to the lattice parameter. Does this help?