Answer:
The net force exerted by these two charges on a third charge is 
Explanation:
Given that,
Third charge 
Distance
Suppose The magnitude of the force F between two particles with charges Q and Q' separated by a distance d. Consider two point charges located on the x axis one charge, q₁ = -12.5 nC , is located at x₁ = -1.650 m, the second charge, q₂ = 31.5 nC , is at the origin.
We need to calculate the total force will be the vector sum of two forces
Using Coulomb's law,

Put the value into the formula


We need to calculate the force will be to the negative charge with opposite charges
Using Coulomb's law,

Put the value into the formula


The force also will be to the negative side, charges with same charge sign
We need to calculate the net force exerted by these two charges on a third charge
Using formula of net force




Negative sign shows the negative direction.
Hence, The net force exerted by these two charges on a third charge is 