So confused Kinematics in one dimension

[PhysicFailure]

A hot air balloon is ascending straight up at a constant speed of 8.80 m/s. When the balloon is 12.0 m above the ground, a gun fires a pellet straight up from ground level with an initial speed of 31.0 m/s. Along the paths of the balloon and the pellet, there are two places where each of them has the same altitude at the same time. How far above ground level are these places?

I got 0.628 and 3.90 for my answers and they're wrong! I don't know what I did but I can't find what I did wrong! help please!

 

[kmikias]

how do you get 0.628 and 3.90 .show us

 

[PhysicFailure]

oh i got the answer! I used this equation and used the quadratic formula:

-4.9t^2+t(Velocity of pellet gun-velocity of balloon)-12=0
then plugged those two answers into the following as is:

0.628 into distance of balloon=12+speed(0.628)= 17.5 m
and distance of pellet=initial speed(3.9)-((9.8(3.9^2))/2)= 46.4 m

 

[kmikias]

Inorder to know where those meet in the same height you have to make equall

like pellet = gun
and you have to solve as you did above