Luis starts at one corner of a square field and walks 80 feet along one and then walks straight back to where he started

The field is square:

if you are looking for its area

A = a² = 80² = 6400 ft²

if you are looking for its perimeter

P = 4 a = 4 (80feet) = 320 ft

All sides of a square have the same measurement. That is why we multiply said measure by 4 to get the perimeter.