Relaxation and recreation, which are considered to be an indispensable element of life.

=Relaxation and recreation and they are considered to be an indispensable element of life.

参考译文：放松和娱乐，这两项被认为是生活中不可或缺的一个元素。 

备注：关系代词代替relaxation and recreation，主语涉及到2种事情，所以用复数are，后面，一种元素，为单数，所以用不定冠词an.类似于“Apples are a kind of fruit 苹果是一种水果” 主语苹果是复数，用复数are，后面表语，a kind of 一种水果，用单数a修饰。