Question 550285
l=4w
2l+2w=50 (this is from the perimeter formula 2l+2w=p

You mentioned you wanted to do this using the elimination method. Let's get both equations in good form for that.
l=4w
subtract 4w from both sides
l-4w=0

2l+2w=50
divide both sides by 2

l+w=25
minus
l-4w=0
5w=25 (minus -4w gives us +4w)
w=5
l=20