Hello Elena You could say this instead. Know sounds better than certain or you could say I beleive, but 'know' is a stronger word. Task is als a better word because 'assignment 'is usually used to talk about the work that you do at school or college I know that he will be able to carry out out any task required of him I know that he will carry out any task with great aplomb I can highly reccomend him as he can carry out any task required Aplomb is quite formal. I am certain that he will be able to perform any assigment ease skilled, thus I strongly recommend him.