Well in this case we would use the distance formula AB = square root (x2-X1)^2 + (y2-y1)^2
In your case you have 2 points where -1 is x1 and 4 is x2, -4 is y1 and 4 is y2.
When you substitute it into the distance formula you get
9.43 units. The working out is in the attachment. Sorry I don't know how to use the formula stuff. If you have any questions ask me.