Solve ∂ 2u ∂x2 + ∂ 2u ∂y2 = 0 for 0 < x < l, y > 0 subject to the following boundary conditions ∂u ∂x(0, y) = 0, ∂u ∂x(l, y) = g(y), ∂u ∂y (x, 0) = 0. if there is a solvability condition, state it and give its physical interpretation.