So you are finding the forces acting on the third charge, for simplicity we'll call it Q3, and the one below it will be Q1, and the negative Q we'll call Q2.
So the formula states that:
Forces acting on Q3 = Force of Q1 on Q3 + Force of Q2 on Q3 or simply:
F3 = F13 + F23
Now just calculate F13 and F23.
The formula for electric force is as follows:
F=(ke * q1*q2)/ r^2
and r is the distance between the two charges.
you will find r for each equation by looking at the diagram.
First lets find F13, which plugging into the equation we get that:
F13 = (ke*2Q^2)/d^2
Now to find F23 is a little trickier and requires breaking down the force into its x and y components.
Lets first find F23(x-component) = (ke*Q^2)(cos45) / (2d^2) (this is found based on the geometry of the triangle)
and similarly F23(y-component) = (ke*Q^2)(sin45) / (2d^2)
Now add the 2 forces, (remember that F13 only has the y-component):
F3 = [(ke*Q^2)(cos45) / (2d^2)] i + [(ke*2Q^2)/d^2 - (ke*Q^2)(sin45) / (2d^2)] j
this can be further simplified to:
F3 = (ke*Q^2) / d^2 [ 1/(2*2^(1/2)) i + (2 - 1/(2*2^(1/2)) j ]
*note the i and j are there to represent the vector components of the force.