Fencing = finding perimeter
Perimeter = Length + Length + width + width
But you are using one of the side of the house , i,e, width at one side doesnt need fencing.
L + L + 60 ≥ 160
2L + 60 ≥ 160
2L ≥ 160 - 60
2L ≥ 100
L ≥ 50
The length must be at least 50 feet long