plz mention velocity "u" of ball is its absolute velocity(relative to ground) or it is velocity relative to ballon.

if it is relative to ground then we can write eq of path of ball as

x=ut-gt²/2

and for ballon

x=-vt

taking downward direction -ve and upward+ve

when the ball will pass by the balloon they will be at same height  let that height be z

putting z in both equations and equating

-vt=ut-gt²/2

so answer shouid be

T=2(V+u)/g