When solving perimeter, remember that you are adding up all the sides to find the distance around an object.
So we know that the longer sides of the blanket is 60, and on a rectangle that the are two sets of equal sides, then we know that the long sides take a up 120 inches out of 210.
To find the other two sides, we subtract the long side from the total.
210-120=90
So the total of the shorter sides is 90
Then we divided by two to find the length of each shorter side.
90/2=45
So the shorter sides are 45 inches in length.
Hope this helps!