Answer:
width = (21 - 2n) ÷ 2 or 21/2 - n
Step-by-step explanation:
To find Width
let's say width is x
so 2x + 2n = 21
take away 2n from both sides
then 2x = 21 - 2n
Divide both sides by 2 to get x on its own
x = 21 - 2n /2
therefore the width of the rectangle is 21 - 2n /2
