A square with area 36 square units has a side of 6 units. A square has 4 sides so its perimeter=4×6=24.
A rectangle with area 36 square units could have 2 sides of length 4 and 2 sides of length 9 units, so its perimeter is 2×4+2×9=8+18=26 units. There are many rectangles with an area of 36 square units and their perimeters are likely to be different.