somebody help me how to find the directtion of velocity?

w(k+1) = (angular(v(k)) - angular(v(k-1))) / t;

v(k) - velocity of object at time k
v(k-1) - velocity of object at time k-1
angular(v(k) - direction of v(k)

I know atan2 but i need more detail exactly.

v(k) found like that:
v(k) = sqrt(((x(k) - x(k-1)^2) + (y(k) - y(k-1)^2))) / t