Question

Firstly , cars add to pollution and poison the air we breath или Firstly ,cars pollute the atmosphere of our planet ,they poison the air we ought to breathe in every day

Answer 1

и здесь первое предложение

Answer 2

Firstly, cars add to pollution and they poison the air we breathe. 
Firstly, every day cars pollute the atmosphere of our planet, they poison the air we  breathe.