[tex]\boxed {\boxed{\text {Perimeter = 2(Length + Width)}}}[/tex]
Given that the perimeter = 15cm and its width is 3.5cm, find the length:
15 = 2(Length + 3.5)
Divide by 2 on both sides:
Length + 3.5 = 7.5
Take away 3.5 from both sides:
Length = 7.5 - 3.5
Evaluate the Right Hand Side:
Length = 4 cm
[tex]\boxed {\boxed{Answer: Length = 4 cm}}[/tex]
