The dimensions of a rectangular garden were 5m by 12m. each dimensions was increased by the same amount. the garden has an area of 120m. find the dimensions of the new garden .

Let's say, the amount that each side was increased was "a"
so, it was 5 wide and 12 long, now it's 5+a wide and 12+a long

we know the area is 120, so whatever "a" is, we know that

(5+a)(12+a) = 120

so   

use those factors, solve for "a"

Are you trying to figure out the volume?