Performing the indicated operations (Definition CVA, Definition CVSM), we obtain the vector equations
\begin{align*}
\colvector{11\\6\\17}
&= 3\colvector{1\\2\\-1} + 4\colvector{2 \\ 0 \\ x}
= \colvector{11\\6\\ -3 + 4x}
\end{align*}
Since the entries of the vectors must be equal by Definition CVE, we have $-3 + 4x = 17$, which leads to $x = 5$.