From the above figure,
Side of square = (3x – 5) m
Perimeter of square = 40 m
⇒4 × side = 40 ⇒ 4 × (3x – 5) = 40
⇒ \(\frac {4(3x-5)}{4} = \frac {40}{4}\) (Divide by 4 on both sides)
⇒ 3x – 5 = 10
⇒ 3x – 5 + 5 = 10 + 5 (Add 5 on both sides)
⇒ 3x = 15
⇒ 3x/3 = 15/3 (Divide by 3 on both sides)
∴ x = 5 m