Explorers on a small airless planet used a spring gun to launch a ball bearing vertically upward from the surface at a launch velocity of 17 m/sec. Because the acceleration of gravity at the planet's surface was gs m/sec", the explorers expected the ball bearing to reach a height of s = 17t-(1/2)gst" m, t sec later. The ball bearing reached its maximum height 20 sec after being launched. What was the value of gs