The sum of the angle measures of a triangle is 180°. So, we write:

\(\displaystyle{m}∠{D}+{m}∠{E}+{m}∠{F}={180}°\)

\(\displaystyle{m}∠{D}+{57}°+{39}°={180}°\)

\(\displaystyle{m}∠{D}+{96}°={180}°\)

\(\displaystyle{m}∠{D}={84}°\)

\(\displaystyle{m}∠{D}+{m}∠{E}+{m}∠{F}={180}°\)

\(\displaystyle{m}∠{D}+{57}°+{39}°={180}°\)

\(\displaystyle{m}∠{D}+{96}°={180}°\)

\(\displaystyle{m}∠{D}={84}°\)