While two forces act on it, a particle of mass m=3.2kg is it to move continuously with velocity (3m/s)i - (4m/s)j . one of the forces is f1 = (2n)i + (-6n)j . what is the other force?

because if the tool that you are using is not balanced it there would be false mass.

it is given that trillions of microbes make up to 1% to 3% of the body mass.

let's take average of this percentage: 2%

consider the average mass of the human body as 70 kg.

then, trillions of microbes would have mass  

therefore, 100 trillion micro-organisms would weigh:

this is an approximate value and could change depending on the mass of the human body and percentage of the microbes present in it.

average mass per microorganism =