#### Answer

He did not keep the x and y values together.

#### Work Step by Step

The distance formula is:
$\sqrt {(x_{2}-x_{1})^{2}+ (y_{2}-y_{1})^{2}}$
As the coordinates are (1,5) and (3,8), the distance formula should have looked like this:
$\sqrt {(3-1)^{2}+ (8-5)^{2}}$
As can be seen, this person did not put their x and y values in the correct spot.