We can see that the length of the blue rectangle is the same as the length of the smaller rectangle = 7 cm.
To calculate the width of the blue rectangle, we take the length of the smaller rectangle (7 cm) and substract two of the width (0.5 cm) of the smaller rectangle:
7 - (2 x 0.5) = 6 cm
Now we have the length and width of the blue rectangle.