How do i figure out if the acceleration is negative, positive or zero ?

At time t=0, a particle is located at the point (3,6,9). it travels in a straight line to the point (5,2,7), has speed 8 at (3,6,9) and constant acceleration 2i−4j−2k. find an equation for the position vector of the particle.

2) why do satelites not fall while revolving around the earth?

Increased ozone in the lower atmosphere near the ground results primarily from