A rectangular garden has a perimeter of 54 inches and a side length of 15 meters. What is the length of the other side?

First of all, there's a problem with the units. 54 inches is less than 1.4 meters,

so the rectangle can't possibly have a 15-meter side. I'm sure you meant that

the perimeter is 54 meters.

One of the other sides is also 15 meters, and the two remaining sides

are both 12 meters. (15 + 12 + 15 + 12) = 54 meters.

The lengths of the other sides is 12 meters!