Verified Answer :

LammettHash

Use the dot product relation:

\(\mathbf u\cdot\mathbf v=\|\mathbf u\|\|\mathbf v\|\cos\theta\)

where \(\theta\) is the angle between \(\mathbf u\) and \(\mathbf v\).

You have

\(\mathbf u\cdot\mathbf v=6\times7+(-1)\times(-4)=46\)
\(\|\mathbf u\|=\sqrt{6^2+(-1)^2}=\sqrt{37}\)
\(\|\mathbf v\|=\sqrt{7^2+(-4)^2}=\sqrt{65}\)

So, the angle is given by

\(\cos\theta=\dfrac{46}{\sqrt{37}\sqrt{65}}\implies \theta\approx20.3^\circ\)