In this paper: (i)we give a short proof of the MacGregor theorem for the radius of convexity of the class P’ of the functions stated in the title and indicate all extremal functions;(ii)more generally, we find the radius of convexity of order alpha of the class P’ with all extremal functions;(iii) we find the radius of convexity of the class P’ when the second coefficient is fixed with all extremal functions; (iv) we compare our results with other results.