Vertically opposite angles are equal. Angles on a straight line add up to 180&deg;.<br><br>
&bull; AB and CD are straight lines crossing at a point<br>
&bull; Read the given angle from the diagram<br>
&bull; Use vertically opposite angles and straight-line property to find m<br>
&bull; Angle m = 21&deg;<br><br>
Final Answer: 21