There are two ways:

First draw the two points and draw a line segment from one to the
other:



Draw a right triangle whose hypotenuse is the green line:



Count the units in the lower red leg of the right triangle and you'll find it
is 6 units long.

Count the units in the right red leg of the right triangle and you'll find it 
is 4 units long.

Use the Pythagorean theorem and find the length of the green hypotenuse:









(4,-1) and (-2,-5).

Another way is to use the formula, which amounts to the same thing:

















Edwin