An ordinary flea has a mass of 210. how many newtons does it weigh?

F=ma
is the formula you need to find the amount of newtons force.
If it weighs 210, and it's being pulled down by gravity at 9.8m/s^2, all you have to do is multiply the two numbers to get 2058.

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

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