A rectangle has the property that the

forms an arithmetic sequence, in that order.

What are the lengths of the sides of the rectangle?

Let the sides of the rectangle be

The area of the rectangle can be calculated by $a(a+d).$

But because the area is the third term in the arithmetic sequence, we also know that the area is equal to $a+2d.$

You can use this to form an equation.

In the same way that we used the area of the rectangle to form an equation, we can use the perimeter to form a second equation.

Once you have both equations, you can solve them simultaneously (try eliminating $d.$)