Recall the following properties of a parallelogram:
1) Angles on the same side of the transversal are supplementary.
2) Opposite sides are equal.
Therefore, for the given parallelogram, it can be deduced that:
[tex]\begin{gathered} m\angle BCD+m\angle ADC=180\degree \\ AB=DC \end{gathered}[/tex]The question provides the following information:
[tex]\begin{gathered} AB=18.3 \\ m\angle BCD=60\degree \\ DC=3x+5 \end{gathered}[/tex]Hence, we have:
[tex]\begin{gathered} m\angle ADC=180-m\angle BCD=180-60 \\ m\angle ADC=120\degree \end{gathered}[/tex]and
[tex]\begin{gathered} AB=DC \\ 18.3=3x+5 \\ 3x=18.3-5=13.3 \\ x=\frac{13.3}{3} \\ x=4.43 \end{gathered}[/tex]ANSWER
[tex]\begin{gathered} \begin{equation*} m\angle ADC=120\degree \end{equation*} \\ x=4.43 \end{gathered}[/tex]