The flea is quite small for its mass to be expressed in grams. I assume that the given mass is in milligrams. Convert this value first to kilograms.
(210 mg) x (1 g/ 1000 mg) x (1 kg/ 1000 kg) = 2.1x10^-4 kg
Multiply this value by the Earth's acceleration due to gravity, 9.8 m/s².
(2.1x10^-4 kg)(9.8 m/s²) = 2.058 x 10^-3 N
