homework and exercises - Why can velocity and acceleration be negative?

Why is speed and acceleration negative when $V_1$ of an object is say 150m/s, $V_2$ is 0 m/s and $\Delta d=0.50\,\rm m$? I found the time it takes which is 0.0033s and the acceleration to be 90909.09 m/s$^2$.

My questions:

1. What do negative velocity and negative acceleration mean?


2. Is it negative because it is slowing down?


3. Why is the acceleration so big? Is that even reasonable?