In each of Problems 23-30, a second-order differential equation and its general solution y(x) are given. Determine the constants A and B so as to find a solution of the differential equation that satisfies the given initial conditions involving y (0) and y' (0).

y" - 10y' + 21y = 0, y(x) = Ae^3x + Be^7x, y (0) = 15, y'(0) = 13