The following was taken from the FINAL CUT PRO X help menu (by typing TRIMMING in the search bar)...
Enter a new duration for a clip using timecode..
You can change the total duration of one or more selected clips by entering a timecode value.
Select one or more clips in the Timeline.
Do one of the following:
Choose Modify > Change Duration (or press Control-D).
Double-click in the center of the Dashboard in the toolbar.
The timecode entry field appears in the Dashboard.
Enter a new duration for the selected clip.
Final Cut Pro moves the end point of the clip to the duration you entered, and any subsequent Timeline clips ripple accordingly.
For more information about entering timecode values, see Navigate using timecode.
Note: If you enter a duration longer than the available media in the clip, Final Cut Pro extends the duration to the maximum length of the clip.
This worked great.
I guess my problem was that I was not searching for the correct info in the help documentation. I should have been typing "trimming".
Also, it took a little playing around to figure out what format Final Cut Pro expects me to type in.
I can't just type 7.5 for 7 1/2 seconds. I needed to type in 7 and then the number of frames to equal half a second.
In my case 7 and then 15 and Final Cut Pro is pretty smart and figures out what I mean. If I type in, say, 32 seconds, it replaces what I type with 1 second and 2 frames for me automatically. Nice.