A small ball of mass m collides with a rough wall having co-efficient of friction $\mu$ at an angle $\mathrm{\theta }$ with the normal to the wall. If after collision the ball moves with angle $\alpha$ with the normal to the wall and the coefficient of restitution is ‘e’, then the reflected velocity $v$ of the ball just after the collision is $v=\frac{u}{\sin \alpha }[\sin \theta -\mu \cos \theta (e+x\left)\right]$ where ‘$x$’ is ____