From the given picture we can see
A small rectangle with a side of length 6 cm
The corresponding side of the large rectangle is 21 cm
To find the scale factor of enlargement we will divide 21 by 6

Part A:
The answer is 3.5
To find the missing side of the small rectangle we will use the scale factor with the corresponding side of the large rectangle which is 52.5 cm

By using the cross-multiplication

Divide both sides by 3.5

Part B:
The missing side in the small rectangle is 15 cm