Hi, there! The question and the correct answer are both shown in the image. However, I haven't had any idea about how to get the correct solution as...

Hi, there! The question and the correct answer are both shown in the image. However, I haven't had any idea about how to get the correct solution as shown. I tried to apply the conversation of momentum and set MaVa,i=MaVa,f+MbVb,f, but that didn't give me the correct solution. Thanks for helping!