Answer
$-2.21\times 10^{-5}C$
Work Step by Step
As given that there is a force of attraction between the charges and one point charge is positive so the required point charge is negative and its magnitude can be determined as follows:
$Q=-\frac{r^2 F}{Kq}$
We plug in the known values to obtain:
$Q=-\frac{(1.31)^2(0.975)}{8.99\times 10^9(8.44\times 10^{-6})}$
$Q=-2.21\times 10^{-5}C$
