Final answer:
The speed of the projectile at the instant it reaches the top of its trajectory is 143 m/s, but in the opposite direction.
Step-by-step explanation:
The speed of the projectile at the instant it reaches the top of its trajectory can be determined using the equations of projectile motion. When a projectile is fired straight upward, its velocity decreases due to the acceleration of gravity until it reaches its highest point, where its velocity becomes zero. Then, it starts falling back down, gaining speed again. At the instant it reaches the top, its velocity is equal in magnitude but opposite in direction to its initial velocity.
In this case, the initial velocity of the projectile is 143 m/s. Therefore, when it reaches the top of its trajectory, its speed will be 143 m/s, but in the opposite direction.