Question

A hot air balloon is traveling vertically upward at a constant speed of 2.8 m/s. When

Answer 1

We can use the equation of motion: 
2as = v² - u²; where s = 29 m, a = 9.8 m/s², u = -2.8 m/s
v = √(2 x 29 x 9.8 + 2.8²)
v = 24. 0 m/s 

Answer 2

why is your formula x=? if i'm reading it right the formula is the height of the object at time t. That being the case, set your setup (which looks right) equal to 0 and solve for t. This will tell you how long it takes to hit the ground. Plug this value of t into the forumla -9.8t+2.8 and that will tell you the speed at time t.