Divide the area by the time:
Divide 2 3/4 acres by 4 1/8 hours:
Divide 2.75 acres by 4.125 hours:
2.75 acres
--------------- = 2.667 acres/hour, or 2 2/3 acres / hour.
4.125 hrs
If this is not entirely clear to you, please share the work you've done yourself and I will respond.