bearing in mind that supplementary angles add up to 180°.
[tex]\bf \stackrel{\measuredangle 1}{(11a+19)}+\stackrel{\measuredangle 2}{(6a-9)}~~=~~180\implies 17a+10=180 \\\\\\ 17a=170\implies a=\cfrac{170}{17}\implies a = 10 \\\\[-0.35em] ~\dotfill\\\\ \stackrel{\measuredangle 2}{6a-9}\implies 6(10)-9\implies 60-9\implies \stackrel{\measuredangle 2}{51}[/tex]
