If perimeter is 42 cm, then x is ________. 

The correct answer is: 3 cm

The perimeter is the sum of the all the sides of the polygon. In this question we are given the perimeter and three sides out of four . We can subtract the sum of the all sides from the perimeter to find the side length x.
Step1:  We are given three sides of the polygon we will add them and subtract from the perimeter to find the fourth side.
Perimeter =
Sum of the three sides = 
=> 
Step4:  Finding the fourth side.

X = .
So, the value of x is