So, I started to think, How could this big factory affect our city positively or negatively?
This sentence does not add much value to your introduction. You better present it with more grip. Tell that it has both positive and negative effects. Then dedicate one body para to talk about positives and the other for the negatives.
a lot of job opportunity
a lot of job opportunitiesStarting from the positive points, the first thing came to my mind, establishing a big factory in our community could offer our city population a lot of job opportunity which apparently will decrease the unemployment percentage that my country suffered from; for the recent decade.
.... this sentence does not have any grammar problems. However, do not lengthen your sentences too much because you would be carried away at the end may tell irrelevant things. Also, the reader needs to remember lots of details and therefore may become exhausted and lose interest.