Final answer:
The negative acceleration of the crate is 5 m/s².
Step-by-step explanation:
The negative acceleration of the crate can be found using Newton's second law of motion, which states that the force exerted on an object is equal to its mass multiplied by its acceleration (F=ma).
In this case, the force of friction acting on the crate is 15N.
Since the force of friction opposes the motion of the crate, it acts in the opposite direction of the acceleration.
Therefore, the negative acceleration can be found by dividing the force of friction by the mass of the crate:
acceleration = force/mass = 15N / 3kg = 5m/s².