Radius of the circle with three coordinate points using Pointers/ without using pointers

Hi,

I want the source code for calculating  the radius of the circle with three coordinate points by using either pointers/ without pointers...

I  have to come out wwith the formula

Formula
Used : 

ya= - 1/ma (x- (x1-x2)/2) + (y1+y2)/2)

yb = - 1/mb(x- (x2+x3)/2) + (y2+y3)/2)

where m is the slope of the line given by 

ma= y2-y1/x2-x1 and  mb= y3-y2/x3-x2

I have only these ponts, i dont knw how to proceed....

Plz help me out...