We are given the following vector
[tex]v=(2,-1)[/tex]The magnitude of a 2-dimensional vector is given by
[tex]|v|=\sqrt{x^2+y^2}[/tex]Substitute the given points into the above formula
[tex]\begin{gathered} |v|=\sqrt{(2)^2+(-1)^2} \\ |v|=\sqrt{4+1} \\ |v|=\sqrt{5} \end{gathered}[/tex]Therefore, the magnitude of the vector (2, -1) is √5
[tex]|v|=\sqrt{5}[/tex]