The main span of Bridge A ​(2300 ​feet) is​ _____ percent shorter than the main span of Bridge B ​(4700 ​feet).

To find this answer you find the percent of change.
To do this you divide the difference in lengths by the starting length. We start with the main bridge.

4700-2300=2400
2400/4700 is about 51.1% shorter.