i might not be correct but i think Perimeter=31. Since the problem tells me that the area is 42 square inches i divided the given area (42 inches) by the given side length (12 inches), and I got 3.5. I did this because the formula for area is a=lw therefore 12×3.5=42 inches. Now that I know the missing measurement of the other side length, I can plug the numbers I have into either one of these equations.
P=SIDE++SIDE+SIDE+SIDE OR P=2(length)+ 2(width). Please let me know whether or not you understand or have any other questions I will be more than happy to answer. Later when you reach 9th grade and become a freshmen you will learn theorems that will help make it a little bit easier and they will also remove A LOT of the algebraic work that's required to solve questions such as this one. My name is Takira and I go to LaGuardia High School for Performing Arts. Please let me know if you have any more questions.