\(m . n = |m||n|\cos \theta\)

\((3i + 4j) . (2i - j) = 6 - 4 = 2\)

\(2 = |(3i + 4j)||(2i - j)| \cos \theta\)

\(|3i + 4j| = \sqrt{3^{2} + 4^{2}} = \sqrt{25} = 5\)

\(|2i - j| = \sqrt{2^{2} + (-1)^{2}} = \sqrt{5}\)

\(2 = 5(\sqrt{5})(\cos \theta)\)

\(\cos \theta = \frac{2}{5\sqrt{5}} = 0.08\sqrt{5}\) 

\(\theta = \cos^{-1} 0.1788 = 79.7°\)