a rectangular garden has a side length [x] and breadth[y].the perimeter of the garden is 18m and its area is 20m squared.calculate the dimensions of the garden

You have:

2x + 2y = 18
x+y = 9
or y = 9-x

xy = 20
x(9-x) = 20
9x - x^2 - 20 = 0
x^2 - 9x + 20 = 0

solve for x, hint: it factors nicely