Answer this question according to chemistry

To calculate the value of $\text{X}$ for the formation of propane, we will use Hess's Law. The target equation is:

$$3\text{C}(\text{s}) + 4\text{H}_2(\text{g}) \longrightarrow \text{C}_3\text{H}_8(\text{g}) \quad \Delta H = \text{X kJ mol}^{-1}$$

We are given the following thermochemical equations:

a) $\text{H}_2(g) + \frac{1}{2}O_2(g) \longrightarrow H_2O(l) \quad \Delta H_a = -280.5