\large\text{$\begin{aligned}&S_n=3n^2-7n\\&{\implies}\:\bold{U_{11}=S_{11}-S_{10}}\quad\left \ \because\ S_{11}=S_{10}+U_{11}\ \right\\&{\iff}U_{11}=3\cdot11^2-7\cdot11-\left(3\cdot10^2-7\cdot10\right)\\&{\qquad\quad\ \ \,}=3\left(11^2-10^2\right)-7(11-10)\\&{\qquad\quad\ \ \,}=3(121-100)-7\\&{\qquad\quad\ \ \,}=63-7\\&{\qquad\quad\ \ \,}=\bf56\end{aligned}$}