if we let y be the dimension of the other side, then the perimeter has the formula
P = 2x + 2y
48 = 2x + 2y
The area of the rectangle has the formula
A = xy
108 = xy
So,
x = 108/y
Substituting this to the first equation
48 = 2(108/y) + 2y
48y = 216 + 2y^2
2y^2 - 48y + 216 = 0
Solving the quadratic equation:
y = 18 or 6
Either of the two is the correct answer for y, the value of x would just be the other. So, the dimensions of the garden is
18 m x 6 m