Notice that:
[tex]\measuredangle ABC=\measuredangle1+\measuredangle DBC[/tex]
Solving for angle 1 we get:
[tex]\measuredangle1=\measuredangle ABC-\measuredangle DBC[/tex]
Substituting the measure of angles ABC and DBC we get:
[tex]\begin{gathered} \measuredangle1=135^{\circ}-75^{\circ} \\ \measuredangle1=60^{\circ} \end{gathered}[/tex]
The measure of ∡1 is 60 degrees.
