This is a conservation of momentum problem. Initial momentum must equal final momentum. Momentum= mass* velecoity I will denote Ball as b and pin as p Initial momentum= V(initial b)* Mass(b)+ V(initial p) *mass (p) since the pin is stationary to begin that means velocity is 0 and we can plug in for the rest Initial momentum= 7m/s*8kg=56 mkg/s Now for final momentum=V(final b)*mass(b)+V(final p)*mass(p) plugin final momentum=V(final b)*8kg+12m/s*2kg we can set final momentum= inital momentum and solve for V(final b) 56mkg/s= V(final b)*8kg+ 24mkg/s 32mkg/s=V(final b)*8kg 4m/s=V(final b) so 4 meters/second is the answer