How to calculate mass of water of crystallisation?
The mass of aluminium sulfate has to be taken along with the water of crystallization.    

Now molar masses
$$
\begin{array}{c| c}
\text{compound} &amp; \text{ mass (g)} \\
\hline
\ce{Al2(SO4)} &amp; 342 \\
\ce{6H2O} &amp; 108 \\
\ce{Al2(SO4)3\cdot 6H2O} &amp; 450\\
\end{array}
$$  

After that it's simply ratio and proportion find out the moles of hydrated salt then find out the moles of water finally get the mass of water. 

$$\frac{13.5}{450} \times 6 \times 18 = 3.42$$