Need some explanation for this physics answer (E-field due to charges)

[ToonBlue]

Why is it that the electric field at Z due to Q1 and Q3 have their horizontal components cancel and also why is the resultant for E1+E3 result in the negative z directiom?

 

[BvU]

Hello TB, welcome to PF !

Please don't delete the template. It's really useful for you to order your thinking and for us to help you in an effective way. We want to help you further and understand, instead of just handing over the answer.

What do you calculate for these horizontal components ?
If those are equal and opposite, then the two z-components are all that remains and they are in the negative z-direction.

 

[DEvens]

Hello ToonBlue. Welcome to the forum.

Ordinarily you should use the template.

Why does the horizontal component cancel? Symmetry.
Why is the result in the negative direction? Sign of the charges. By this convention the electric field points towards -ve charges and away from +ve charges.