The original area is ab.
When the length of both sides is increased by 1/3, the length is (1+ 1/3)a and the width is (1+1/3) b. 。
Then the new lawn area is (1+1/3) a× (1+1/3) b.
=4/3 a × 4/3 b
= 16/9 ab
16/9 ab - ab = 7/9 ab
The area of the new lawn has increased by 7/9 ab ÷ ab = 7/9.
I hope it helps you!