Physics Doubt

https://preview.redd.it/dbhjv6wv2vvd1.png?width=640&format=png&auto=webp&s=34dc7d27ccf888de64119caaed19f7191c243cc5

https://preview.redd.it/7nng494x2vvd1.png?width=634&format=png&auto=webp&s=8e2517a97cff887af9b336a733b1bdd0998a41fc

Could Anyone help me to solve these 2 questions. I do not understand the answer given by the board. Please do it fast as i have an exam tomorrow