Sunlight near the Earth has an intensity of 1361 W/m²…

Step 1. The black panel. $p = \dfrac{I}{c} = \dfrac{1361\ \text{W/m}^2}{3.00 \times 10^{8}\ \text{m/s}} = 4.54\ \mu\text{Pa}$.

Step 2. The mirror. $p = \dfrac{2I}{c} = 9.07\ \mu\text{Pa}$.

Step 3. The force on the black panel. $F = pA = 4.54 \times 10^{-6}\ \text{Pa} \times 3.0\ \text{m}^2 = 13.6\ \mu\text{N}$.