To find the scale factor used to enlarge the rectangle, you can compare the corresponding side lengths of the small rectangle and the big rectangle.
For the length:
- Small rectangle length: 5 inches
- Big rectangle length: 9 inches
Scale factor for length: \[ \text{Scale Factor} = \frac{\text{Big rectangle length}}{\text{Small rectangle length}} = \frac{9}{5} = 1.8 \]
For the width:
- Small rectangle width: 3 inches
- Big rectangle width: 5.4 inches
Scale factor for width: \[ \text{Scale Factor} = \frac{\text{Big rectangle width}}{\text{Small rectangle width}} = \frac{5.4}{3} = 1.8 \]
Both the length and the width give the same scale factor of 1.8.
Therefore, the scale factor used to enlarge the rectangle is \( \boxed{1.8} \).