To solve this problem it is necessary to apply the concepts related to the conservation of the momentum for an inelastic shock. Mathematically this can be described as
![m_1v_1 + m_2v_2 = (m_1+m_2)V_f](https://img.qammunity.org/2020/formulas/physics/college/bteam194tchhtexag05xv7pq1jlu68ebv5.png)
Where,
m_{1,2} = Mass of each object
v_{1,2} = Initial velocity of each object
V_f = Final Velocity
If we assign the value of mass 1 and speed 1 to the cart and the variable of mass 2 to clay (which is at rest) we will have:
![m_1v_1 + m_2v_2 = (m_1+m_2)V_f](https://img.qammunity.org/2020/formulas/physics/college/bteam194tchhtexag05xv7pq1jlu68ebv5.png)
![(306)(14.2) + (76.3)(0) = (306+76.3)V_f](https://img.qammunity.org/2020/formulas/physics/college/fijfvhrovn5j2tkl9753cppdd4kb431al0.png)
![V_f = 11.36cm/s](https://img.qammunity.org/2020/formulas/physics/college/4xtj5ambhbwdb10kmumv2agtg7trpuzp6t.png)
Therefore the final speed of the system would be 11.36cm/s