Answer
$p_r = 1.7\times 10^{-11}~Pa$
Work Step by Step
If unpolarized light is transmitted through a polarizing sheet, then the transmitted intensity is half of the initial intensity.
The transmitted intensity is $I = 5.0~mW/m^2$
Therefore the intensity of light that was absorbed by the polarizing sheet is $I = 5.0~mW/m^2$
We can find the radiation pressure on the sheet:
$p_r = \frac{I}{c}$
$p_r = \frac{5.0\times 10^{-3}~W/m^2}{3.0\times 10^8~m/s}$
$p_r = 1.7\times 10^{-11}~Pa$
