Hello,
2(l+w)=78
Divided 2 from both side
2(l+w)/2=78/2
l+w=39 feet
l=6w+4
Replace l with 6w+4
6w+4+w=39
7w+4=39
Subtract 4 from both side
7w=35
Divided 7 from both side
w=5 feet
l=6w+4
l=6(5)+4
l=30+4
l=34 feet. As a result, the length of the rectangle is 34 feet and the width of rectangle is 5 feet. Hope it help!