Let...
l=length
w=width
Because we know that the perimeter is 90, we can make this equation...
2l+2w=90
Because we know that the length is 5 feet less than 4 times the width, we can make this equation...
l=4w-5
We can solve this system through substitution...
2(4w-5)+2w=90
8w-10+2w=90
10w=100
w=10 ft
l=4(10)-5=35 ft