\displaystyle P = I^2 R,\quad 0.25m = \left( \frac{12}{R_a + 40k} \right)^2 \times 25k,\quad \frac{12}{R_a + 40k} = 0.1m,\quad R_a = 80k\Omega
\displaystyle P = \frac{V^2}{R},\quad 2.4m = \frac{12^2}{R_b + 40k},\quad R_b = 20k\Omega
R_a - R_b = 60k\Omega