Answer: 16 m by 12 m.
Explanation:
Given: The length of garden l = 13 m
The width of the garden w= 9 m
The area of the rectangular garden is given by :-

It she wants to increase the garden, the dimensions of the new garden will be (13+x) m by (9+x) m , then according to the question we have :-

Since,

Hence, x=3
Therefore, the dimensions of the new garden will be (13+3) m by (9+3) m=16 m by 12 m.