Answer
Magnitude: $65.2 N$
Direction depends on the position of the charge
Top left: North West
Top right: North East
Bottom left: South West
Bottom right: South East
Work Step by Step
The charges can be treated as point charges. Because each charge has the same magnitude and is placed at different corners of a square, all charges will feel a force of equal magnitude. Use Coulomb's Law to find the magnitude.
$F=\frac{kQ^{2}}{r^{2}}$
First, lets find the magnitude of the force of the top right and bottom left charges on the top left charge
$F=\frac{(8.99\times10^{9}\frac{N\times m^{2}}{C})(6.15\times10^{-6}C)^{2}}{(0.100m)^{2}}=34.0N$
Now lets find the magnitude of the force of the bottom right charge on the top left charge. Because the two charges are on the opposite vertices of the square, the distance is
$\sqrt (0.100m^{2}+0.100m^{2})=0.141m$
$F=\frac{(8.99\times10^{9}\frac{N\times m^{2}}{C})(6.15\times10^{-6}C)^{2}}{(0.141m)^{2}}=17.1N$
Now we need to find the total magnitude.
$F_{net}=\sqrt (34.0N^{2}+34.0N^{2})+17.1N=65.2N$
Because all the charges are positive, the direction of this force is North West, away from the other charges. The direction of the force on the top right charge will be North East, the direction of the force on the bottom left charge will be South West, and lastly, the direction of the force on the bottom right charge will be South East.