$${P = \frac{U^2}{R} = \frac{U^2}{R_1 + R_2} = \frac{U^2}{\frac{U^2}{P_1} + \frac{U^2}{P_2}} = \frac{1}{\frac{1}{P_1} + \frac{1}{P_2}} = 6\,\mathrm{W}}$$