There are two ways to do this:

Plot the points and connect them with a line segment:



Then we draw in these two line segments forming a right
triangle:



We count how many units long the bottom (horizontal) side of 
that right triangle is and see that it is 9 units long. Then 
we see that the left (vertical) side of the right triangle 
is only 1 unit long. And the hypotenuse is the distance
between the desired points.  






So we use the Pythagorean 
theorem 








Another way is to memorize the formula



and plug in , , , 









Either way we get the same answer.

The distance is 

Edwin