500
Put the x coordinate of the midpoint on one side of an equal sign then put the x coordinate of the endpoint on the other side add it to a variable then divide that side by 2, times the equation by 2 then subtract the number on the variable side and the variable will be the x coordinate of the other endpoint, do this again for the y coordinates.
How will you find the second endpoint if all you have is the first endpoint and a midpoint?