Vertical throw initial height 15 time 3s until hiting ground

[Justas]

Homework Statement

Height - 15m
Time when ball hits the ground after upward throw 3 s.

I have been trying to find intitial speed with these formulas :
d=v_initial*t-0.5*g*t^2
and
v=v_initial+g*t

Please help to understand how to find initial speed and after what time the ball is at 15m height again.

 

[PeroK]

Homework Statement

Height - 15m
Time when ball hits the ground after upward throw 3 s.

I have been trying to find intitial speed with these formulas :
d=v_initial*t-0.5*g*t^2
and
v=v_initial+g*t

Please help to understand how to find initial speed and after what time the ball is at 15m height again.

You need to post what you've done. Hint: what do you know about quadratic equations?