Find the solution u(x, y) of Laplace's equation in the rectangle 0 < x < a, 0 < y < b, that satisfies the boundary conditions u(0, y) = 0, u(a, y) = 0, 0 < y < b, u(x, 0) = 0, u(x, b) = g(x), 0 ≤ x ≤ a.