Can someone explain me how to get this answer?

Question

A dynamite blast at a quarry launches a chunk of rock straight upward, and 1.9 s later it is rising at a speed of 20 m/s. Assuming air resistance has no effect on the rock, calculate its speed at (a) at launch and (b) 4.1 s after the launch.

Answer 1

20= Vyo -9.81(1.9) yields Vyo= 37.3 m/s. Vy(4.1)=37.3-9.81*4.1= -2.93 m/s