after the car accident, what does gatsby decide to do that he has't done all summer?

1) he wants to use the pool and go for a swim

2)

a) "About three o’clock the quality of Wilson’s incoherent muttering changed — he grew quieter and began to talk about the yellow car. He announced that he had a way of finding out whom the yellow car belonged to, and then he blurted out that a couple of months ago his wife had come from the city with her face bruised and her nose swollen."

1st she's come hime after being beaten

b) "Michaelis opened the drawer nearest his hand. There was nothing in it but a small, expensive dog-leash, made of leather and braided silver. It was apparently new."

She'd been receiving gifts

c) “I know,” he said definitely, “I’m one of these trusting fellas and I don’t think any harm to nobody, but when I get to know a thing I know it. It was the man in that car. She ran out to speak to him and he wouldn’t stop.”

She'd run after a man in a yellow sports car

It never directly says whci reason made him sure; it was a combination of all of them.