That isn't a division sign. It's a radical. Take the square root.
Okay, you've found the center of a circle to be ( X=12.4 , Y=11.8 ).
The true position of the circle should be ( x=12, y=12 ).
How far is your circle from the true position of (12,12) ?
I'll use ^2 to indicate taking the square of a number. For example , 3^2 = 9.
d = square root ((xa-xt) ^2 + (ya - yt)^ 2)
Now plug your values in:
d= square root ( (12.4-12)^2 + (11.8-12)^2 )
d= square root ( ( .4 ^2) + (-.2 ^2) )
d= square root ( .16 + .04 )
d = square root ( .2 )
d = .4472