Okay, so P = perimeter, and the problem says P = 50
Let's call x the width, and y is the length.
In the perimeter formula, 2X + 2Y = P
so 2X + 2Y = 50
The problem also tells us that that x = y+5, so now let's do some substituting and replace X with Y+5
2(y+5) + 2Y = 50