The measure of an exterior angle is equal to half the difference of the measure of intercepted arcs. For the given extrior angle:
[tex]m\angle C=\frac{1}{2}(\text{mAE}-mDB)[/tex]Use the equation above to solve mDB:
[tex]\begin{gathered} 25=\frac{1}{2}(100-mDB) \\ \\ (2)25=(100-mDB) \\ \\ 50=100-mDB \\ \\ 50-100=-mDB \\ \\ -50=-mDB \\ \\ -50(-1)=mDB \\ \\ \text{mDB}=50 \end{gathered}[/tex]