prove using properties of determinants:

Xc1 Xc2 Xc3

Yc1 Yc2 Yc3 = xyz/12(x-y)(y-z)(z-x)

Zc1 Zc2 Zc3

 take x,y,z from R1,R2 and R3 RESP.

operate R2-R1 and R3-R1 and take y-x from R2 and z-x from R3 and expand the remaining det