Hey, how do you know in a random question with 2 coordinates which is the x1 and y1 and which is the x2 and y2? I keep doing the x1/x2 and y1/y2 the wrong way round!

E.G. The line MN is the diameter of a circle, where M and N are (6,-4) and (0,-2) respectively. Find the radius of the circle

I know the forumla being the square root of (x2-x1)^2 + (y2-y1)^2...

But how on earth do you know from the question which is y2 and y1? I keep getting it wrong!

Thanks