Physics problem, please help.

I know that this may not be the appropriate place to ask but I don't know to who other ask. Sorry in advance if is too much off topic.
I have this physics problem that I can't solve, it's very basic, I think, but i'm bad at doing it I tried all the day and tried to look something similar but i can't find the solution(I suck).

So, i have a figure (the link is below, with also the text of the exercise in the original language), I have 4 electric charges (q, 2q and so on) at distance (a) between them in the square and I have to find magnitude and direction of the electric field at the point where the q charge is in function of (q) and (a) and of the force on that change.
Image

I tried to calculate the magnitude of electric field on the 3 charges E2 = ((1/(4πε)) ((2|q|)/a^2)) E4 = ((1/(4πε)) ((4|q|)/a^2))
E3 = ((1/(4πε)) ((3|q|)/((a sqrt(2))^2)) and sum them but my solution is different from the one I got so i think my way to solve it it's wrong, and I'm stuck here.

The solution: here

Can you help me? sorry if the terms i used are not correct I tried to translate them.