If you throw a ball against the ceiling so the ball moves upward and then rebounds to move downward at the instant the ball hits the ceiling, the acceleration is:

Negative

Explanation:

Because the ball is acting under the influence of gravity. In other word,it is moving against gravity.

 Since no heat is lost to the surroundings, the change in thermal energy of the brass piece and the water sample have have the same magnitude: 

m_brass ∙ c_brass ∙ ∆T_brass = m_water ∙ c_water ∙ ∆T_water 

=> 

c_brass = c_water ∙ (m_water/m_brass) ∙ (∆T_water/∆T_brass) 
= 4186J/kg°C ∙ (2.8kg/0.59kg) ∙ ((6.8°C - 5°C) / (98°C - 6.8°C)) 
= 392J/kg