[問3]To watch TV is fun.(Itで始まる文に)
[問5]Making cakes is not easy for her. (It で始まる文に)
[答5]It is not easy for her to make cakes.
[問6]To speak with your mouth full is not good for you.(Itで始まる文に)
[答6]It is not good for you to speak with your mouth full.