Can I cancel a signed contract for financing new car?

The reasons I want to cancel because the contract was signed under pressure - the auto dealer moves the car in front of me and said "you can drive home now when you put money down..." without showing me the car's window sticker, On the contract, the length of loan was changed to 84 mos. from 72 without my consent. They added a big hidden amount $3,500 11.4% of sticker's price without telling me the additional cost was charged to buy a lower rate from lender in behalf on me. ..