Hello!
Remember what the question tells us- that the shape is a square, and one side has the side length of 13 feet.
Recall what a square is: a shape with four sides, with all four sides having the same length.
Addressing the definition of a square- All four sides have the same length, and remember that the question states a square has a side length of 13 feet. Therefore, if you connect the two, you can realize all four sides have a length of 13 feet.
Now to calculate the area. How would you do that? The formula is length * width, but why does that work?
Let's take the question you have posed to answer that. We now know that the length and width of the square are both 13 (as the length and the width are, in a rectangle, the pairs of side lengths with the same lengths). To see why the formula works, I would imagine it on a visual scale.
Imagine a chunk, length 13 feet by width 1 foot. To make it a square with a width of 13 feet, I would keep adding the line over and over again, until you do it 13 times, making a square with the dimensions 13 by 13. That is essentially what you do in multiplying, and is why when you multiply two side lengths, it will make a two dimensional-shape with those dimensions.
But we still don't know why it gives the area. To find out why, return to the visual imagination. We've already seen how it makes a square, so now we can think about how that gives us the area. Think about the square. Now, the way to look at this is to think the dimensions are splitting up the square into different, 1 square foot, chunks. 13 sections made lengthwise, 13 made width wise. This works for essentially the same concept- the fact you are taking 13 and layering it 13 times.
Now, let's solve.
(My answer to the, why does this work, question, was fairly vague, there are many other reasons why it works, I personally would look into it further if it interests you!)
We can use the length * width formula. We know it works, and we know the length and width (both 13), so if you plug them both in, then you get 13 * 13.
The only thing left to do is solve. 13 * 13 = 169, adding on the unit, would make it 169 square feet, your solution.
Hope this helped!
