You insert the numbers into the formula, which gives you $$\begin{array}{llll}\hfill \cos \alpha & =\frac{\left(4,1\right)\cdot \left(2,-3\right)}{\left|\left(4,1\right)\right|\cdot \left|\left(2,-3\right)\right|}& \hfill & \\ \hfill & =\frac{8-3}{\sqrt{4^2+1^2}\cdot \sqrt{2^2+{\left(-3\right)}^2}}& \hfill & \\ \hfill & =\frac{5}{\sqrt{17}\cdot \sqrt{13}}& \hfill & \\ \hfill & \approx \frac{5}{14.86}& \hfill & \\ \hfill & \approx 0.336.& \hfill & \end{array}$$

That means the angle is