Perimeter of the parallelogram = 54 inches
Length of one side of the parallelogram (a) = 18 inches
Let us assume the length of the adjacent side (b) = x inches
Now we already know
Perimeter of a parallelogram = 2 (a + b)
54 = 2(18 + x)
54/2 = 18 + x
27 = 18 + x
x = 27 - 18
= 9
So the length of a side adjacent to 18 inches side is 9 inches.