Final answer:
To find the amount of border Kim needs for her bedroom walls, we can use the expression 8x, where x represents the length of the shorter walls.
Step-by-step explanation:
To find how much border Kim needs, we need to determine the lengths of the walls. Let's assume the shorter walls have a length of x. According to the problem, the longer walls are 3 times the length of the shorter walls, so their length would be 3x.
To find the total border needed, we add up the lengths of all four walls. The border for the shorter walls would be 2x (since there are two of them), and the border for the longer walls would be 2 times 3x, which is 6x.
Therefore, the expression to find how much border Kim needs is 2x + 6x, which simplifies to 8x.