\[x^{2} - 4x + 2 = 0\]
\[x_{1}^{2} + 3x_{1}x_{2} + x_{2}^{2} =\]
\[= \left( x_{1} + x_{2} \right)^{2} + x_{1}x_{2} = 4^{2} + 2 =\]
\[= 16 + 2 = 18.\]