Hey there!
So we know the perimeter is 20 cm. Let's make our formula for this.
2L + 2W = 20
The formula for the area of a rectangle is L x W. Let's make this into our formula and fill it in as time goes.
L x W
Our Length is measured at 7 cm, so let's fill that into our formulas.
2(7) + 2W = 20
7 * W = x
Let's use some basic algebra to solve for our perimeter.
2(7) + 2W = 20
14 + 2W = 20
2W = 6
W = 3
This means our Width is 3. Now all we have to do is solve for our area since we have our dimensions.
7 x 3 = 21
The area of the rectangle is 21 cm^2.
I hope this helps!