C1 Help please, don't understand answer in book!

Hello

I was happily doing some maths until i looked at the answer

I cant understand it.

The question was

Points A and B have co-ordinates (-1,2) and (7, -4) respectively.

a) Write down the Coordinates of M, the midpoint of AB. I got (3,-1)
b) Calculate the distance MB, I got 5
c) The point P lies on the circle with AB as diameter and has coordinates, (2,y) where y is positive. Calculate the value of y giving your answer in surd form.

My answer was 2root 6 But the answer was 2root 6 - 1

My working:

As (3,-1) was the mid point, and the distance to A and B was 5, the distance from (3, -1) is also 5.
I then drew a little triangle.

with the hypotenuse being 5, ( the radius) along the bottom, 1, as the centre of the circle is (3, -1), and we are trying to get to (2,y) so moved one along the X axis. Then I calculated the other side, which is root 24, then i simplified it to 2root 6.

But this is wrong?!?! Help? Thanks

The other side is root 24 but this is not y, this is y+1. Remember the y coordinate of the centre is -1.