I assume you mean "If the *length* of a rectangle is 3 times its width". I will go with that assumption to help you out here.
So let's call the width "W". And since we know the length is 3 times the width, let's call the length "3W". Since it's a rectangle and opposite sides are equal, you know that two sides will be W long and the other two sides are 3W long. Your equation will look like this:
W + W + 3W + 3W. And since we know the perimeter is 16ft, it will be W + W + 3W + 3W = 16.
Add all the like "W" terms together to get 8W = 16ft. Divide by 8 on both sides and you know the width is 2ft. Plug in the 2 into the 3W go get the length and you get the length as 6ft.
But wait, the question asks square feet which indicates area. You just simply multiple the length (6) by the width (2). I'll let you do that yourself. Be sure your answer is in the right units ("sq ft" or "ft2")
I hope this helps. If you need further assistance, please do not hesitate to reach out. I can help through video conference! Good luck!