Hess’s Law of Constant Heat Summation

Calculate the amount of heat necessary to rise $180 \mathrm{g}$ of water from $25°\mathrm{C}$ to $100°\mathrm{C}$. Molar heat capacity of water is $75.3 {\mathrm{Jmol}}^{-1}{ \mathrm{K}}^{-1}$.

Define Hess's law of constant heat summation.

Given that  ${\mathrm{C}}_{\left(\mathrm{s}\right)}+{\mathrm{O}}_{2\left(\mathrm{g}\right)}\to {\mathrm{CO}}_{2\left(\mathrm{g}\right) }; ∆\mathrm{H}°=-\mathrm{a} \mathrm{kJ}$ ;

$2{\mathrm{CO}}_{\left(\mathrm{g}\right)}+{\mathrm{O}}_{2\left(\mathrm{g}\right)}\to 2{\mathrm{CO}}_{2\left(\mathrm{g}\right)} ; ∆\mathrm{H}°=-\mathrm{b} \mathrm{kJ}$.
Calculate the $∆\mathrm{H}°$for the reaction ${\mathrm{C}}_{\left(\mathrm{s}\right)}+\frac{1}{2}{\mathrm{O}}_{2\left(\mathrm{g}\right)}\to {\mathrm{CO}}_{\left(\mathrm{g}\right)}$.