Complex sentence: She became Director General of the National Trust in 2001 and during her tenure membership of the charity, which looks after of land in the United Kingdom, grew from 2.7 to 4 million people.
Split into simpler sentences: She became Director General of the National Trust in 2001. During her tenure, membership of the charity, which looks after of land in the United Kingdom, grew from 2.7 to 4 million people.