Molarity of ${\mathrm{H}}_2{\mathrm{SO}}_4$ is 18M. Its density is $1.8\mathrm{g}/{\mathrm{cm}}^3$, hence molality is :

1 L of ${\mathrm{H}}_2{\mathrm{SO}}_4$ corresponds to $1800 \mathrm{g}$ or $1.8 \mathrm{kg}$.

Out of this $18\times 98=1.764 \mathrm{kg}$ corresponds to ${\mathrm{H}}_2{\mathrm{SO}}_4$.

Hence, the mass of water is $1.8 \mathrm{kg}-1.764 \mathrm{kg}=0.036 \mathrm{kg}$
Hence, the molality of the solution is $18/0.036=500 \mathrm{m}$.