Problem: What is the net force exerted by these two charges on a third charge q3= 45.0nC placed between q1 and q2 and at x3=-1.210m?

As the question states, we use Coulomb's law to get the electric force as:

$\boxed{\mathbf{F}\mathbf{=}\frac{\mathbf{k}{\mathbf{q}}_{\mathbf{1}}{\mathbf{q}}_{\mathbf{2}}}{{\mathbf{r}}^{\mathbf{2}}}}$, where F is the electric force, k is Coulomb constant, and q₁ and q₂ are charges separated by distance r.

The distance of separation from charge 1 to charge 3 is:

r₁₃ = - 1.650 - (-1.210) = - 0.440 m

The electric force that q₁ exerts on q₃ is: