7. Solve the problem of the potential equation in the rectangle $0 < x < a$, $0 <$
y < b, for each of the following sets of boundary conditions. Before solving,
make a pictorial version of the problem as in Exercise 9 of Section 4.1
c. u(x, b) = bx, 0 < x < a; u(a, y) = ay, 0 < y < b; u = 0 on the other two
sides of the rectangle.