Imagine that you fire a toy rocket straight up into the air. As shown in following image, its engine burns for 8.0 s before it runs out of fuel. The rocket continues to climb for 4.0 s, then stops and begins to fall back down. After falling freely for 4.0 s, a parachute opens and slows the descent. The rocket then reaches a terminal velocity of 4.0 m/s [down].

Determine the instantaneous velocity at t = 7 s, t = 13 s, t = 21 s, and t = 25 s. Hint: Draw the tangent line at the specific time and calculate the slope.