Question 295583
Ok, 
Length = x
Width = x - 10

Perimeter = 2(L + W) <br>
We substitute "x" for length, and "x - 10" for width:
Perimeter = 2(x + (x - 10))<br>
Now distribute:
Perimeter = 2x + 2x - 20 Combine like terms:
Perimeter = 4x - 20 & bob's your uncle!