You need to put the given information into both those formulas.

For the formula , you have and , which tells you that (it's negative because the car is decelerating, so is negative).

For the formula , you have , and . Plug those into the formula and you get . I have written the last term as , because you can now substitute in the value for that you got from the previous equation. You then have a simple equation for , and once you know you can put it into the formula to find .