Mixed problems are changed to boundary value problems by making used of Laplace transform. In classical boundary value problems, boundary conditions are local, but boundary conditions could be global ([2, 3, 11]). Every boundary value problem depends on a potential method in mathematical-physics theory. Of course, one couldn’t solve some many problems by potential methods. We shall give a method in which one could reduce every boundary value problem to the second kind Fredholm integral equations and then solve it. In fact, we could obtain solution of every local, non-local or global boundary value problem by this method. Finally, we shall give some sufficient conditions for existence of solutions of the Fefferman’s problem A ([4]).