Calculate acceleration from velocity change and time.
Acceleration is the rate at which velocity changes, so it is the difference between final and initial velocity divided by the time taken. Because the acceleration is uniform, the distance travelled follows from the average of the two velocities rather than needing calculus, and Newton's second law converts the acceleration into the net force the mass must experience to produce it.
Uniform acceleration
a = (v - u) / t; distance = (u + v) / 2 x t; F = m x a
It means the velocity is decreasing in the direction you defined as positive. For an object moving forward, negative acceleration is deceleration; the sign is a statement about direction, not about slowing down as such.
Under constant acceleration the velocity-time graph is a straight line, so the area beneath it — which is the distance — equals the mean of the start and end velocities multiplied by the time. That is exact, not an approximation.
No. The net force is what remains after drag, rolling resistance and gravity components are subtracted, so a vehicle's engine must produce more than this figure to achieve the acceleration.