Chapter 5 - Newton's Laws of Motion - Problems and Conceptual Exercises - Page 144: 61

$9.1$ kilograms

We know that $a=\frac{2x}{t^2}$ $\implies a=\frac{2(2.29m)}{(3s)^2}$ $a=0.509m/s^2$ Now the mass of the dog food can be determined as: $m_d=\frac{F}{a}-m_c$ We plug in the known values to obtain: $m_d=\frac{12N}{0.509m/s^2}-14.5Kg$ $m_d=9.1Kg$