Since ABCD is a trapezoid, notice that BC is parallel to AD.
CD would be a transversal through the pair of parallels, BC and AD. Therefore, the angles ∠C and ∠D are supplementary angles.
Therefore:
[tex]\begin{gathered} m\angle C+m\angle D=180 \\ \Rightarrow121+m\angle D=180 \\ \Rightarrow m\angle D=180-121 \\ \Rightarrow m\angle D=59 \end{gathered}[/tex]
