According to the given question we have the following:
The area of a square backyard is 79 ft
Therefore if the area of a square backyard is 79 ft that means that length of each side would be the following:
length of each side=area of a square backyard/2
length of each side=79 ft/2
length of each side=39.5
Therefore if 39.5 is lenght of each side, the perimeter of the backyard would be calculated as follows:
perimeter of the backyard=l+l+l+l
perimeter of the backyard=39.5+39.5+39.5+39.5
perimeter of the backyard=158