If you sell the 75 call and it's assigned, you will be short the stock at 75. Your $2 loss comes from selling the stock at $77 (less the original premium received). As advised by others, read as much as u can before diving in.
I am surprised that no has pointed out that selling a naked call is considered the riskiest option trade. Your loss is potentially unlimited. Like the coach says, read a book or two first. Also, you first need to fill out your broker's form for adding option trading to your account. If they let you sell naked calls with zero knowledge or experience, maybe you can sue them.