Finding axis of a cylinder I have to find axis of a cylinder that has the top in the origin and the points $A(-5,6,-4),B(-4,-1,2),C(-1,2,4)$ lie on its lateral area. 
Now I know that points A,B,C have the same distance to the axis, but I don't know how I could find this distance ( radius). 
I would really appreciate any help. 

So, the axis is through the origin. Place a unit vector there (two variables to fix) and impose that its cross-product with the position vectors for $A,B,C$ be of the same modulus, which will be distance $r$.
