I don't know the distance formula off the top of my head, so I'll draw a triangle and use the pythaorean theorem. This is what the points look like when plotted. Draw a line that connects them (will be hypotenuse) and draw two legs that connect to make a triangle. After you draw the legs, find their length and input it into the pythagorean theorem.
a² + b² = c²
4² + 8² = c²
16 + 64 = c²
80 = c²
The distance (hypotenuse) is √80 or approximately 8.95, rounded to 9 units.
Hope this helps!