Doubts on the sign of acceleration

[Like Tony Stark]

Homework Statement

Let' suppose that a problem says "a car has a velocity of $5 m/s$ and an acceleration such that it stops some seconds later". And I want to solve for the acceleration so I can replace it in another equation.

Relevant Equations

$V_f =v_0 +a.t$

What should I do? Because I have two possibilities. I have $0=5+at$ so $-5/t =a$. But then I can also say that the acceleration is a negative because it is stopping, so I can write it like $0=5-kt$ and then $5/t =k$

 

[PhanthomJay]

In case 1 you get a neg value for a which means it was not positive per your assumption in the equation, but rather it is a negative. In case 2 you assumed a was neg and when your ans came out positive, it means you assumed the correct sign for a. Bottom line is when your answer comes out negative, you assumed the wrong sign (direction) ; when it comes out positive, you assumed the right sign.
Ahhhh...the wonders of the minus sign...