Answer: g = -3.143 m/s²
Explanation: To determine acceleration due to the gravitational force, it can be used the following formula:
v = v₀ + gt, where:
v₀ is the initial velocity;
g is acceleration due to gravity
t is the time to return to the point of origin
When the rock return to the point of origin, there is no velocity, so v = 0.
The time to go up is the same to go down, so t =
=
.
Substituing in the formula:
v = v₀ + gt
0 = 11 + g.
g = 
g = - 3.143
The acceleration due to the gravitational force is g = - 3.143 m/s².