We have two components of acceleration of point P- one in radial direction which is r \\omega^2 and the other in tangential direction which is r\\alpha . Also, every point on the disc has a forward acceleration of a. Thus we have total acceleration in the forward direction as a+r\\alpha .Thus we get the resultant acceleration of point P as \\sqrt{(a+r\\alpha)^2+(r\\omega^2)^2} .