Answer
$21.28^0$
Work Step by Step
From the law of cosines
$4^{2}=11^{2}+10^{2}-2\times 11\times 10\times \cos \theta \Rightarrow \cos \theta =\dfrac {11^{2}+10^{2}-4^{2}}{2\times 11\times 10}$
$=\dfrac {41}{44}\Rightarrow \theta =\cos ^{-1}\left( \dfrac {41}{44}\right) \approx 21.28^{0}$
