Answer
5 units
Work Step by Step
We know that the distance $d$ between two points $(x_{1},y_{1})$ and $x_{2},y_{2})$ is given by $d=\sqrt ((x_{2}-x_{1})^{2}+(y_{2}-y_{1})^{2})$.
Therefore, the distance between $(5,1)$ and $(8,5)$ will be $d=\sqrt ((8-5)^{2}+(5-1)^{2})=\sqrt ((3)^{2}+(4)^{2})=\sqrt (9+16)=\sqrt 25=5$ units