First of all, I would like to apologize for the question I'm gonna ask regarding the fact, that I am absolute beginner in statistics.

Currently I'm trying to test some hypotheses in Rstudio using Simple Linear Regression, as I am aware that before interpreting results of my model I have to provide verification of assumptions that should be meet within the model.

Unfortunately, one of the key assumption is not very clear for me, could someone explain please?

*"The mean of the residuals should be equal to zero."*

I have no clue which of the residuals I should sum and then divide by number of items. I tried every possibility but it's never zero (I tried it also within other models). So basically my conclusion is that I'm doing something wrong.

I tried to plot residuals and it seems fine for me.

I would be very thankful for any advice or explanation.

Thank you very much.