OK, what we are facing is a kinematic question.
First job, list what you know in a neat table with units, put a question mark next to the thing you’re looking for and a dash next to the factors that are not mentioned:
s = ?m (displacement)
u = 15.9m/s (initial velocity)
v = 0m/s (final velocity, it stops at the highest point)
(note the minus sign? It shows the acceleration vector is pointing in the opposite direction to the positive velocity vector)
t = -s
So we need one of the kinematic equations linking s, u, v and a.
There is only one:
A key understanding here is that the final velocity in the y-direction will be zero.
First, calculate the time the stone will be in the air
d = 12.88 m