Two uniform smooth spheres \(A\) and \(B\) of equal radii have masses \(2 m\) and \(m\) respectively. Sphere \(B\) is at rest on a smooth horizontal surface. Sphere \(A\) is moving on the surface with speed \(u\) and collides with \(B\). Immediately before the collision, the direction of motion of \(A\) makes an angle \(\alpha\) with the line of centres of the spheres, where \(\tan \alpha=\frac{4}{3}\) (see diagram). The coefficient of restitution between the spheres is \(\frac{1}{3}\).

Find the speed of \(A\) after the collision.