I'll do it the easy way,,,one person stand at the net with a stop watch(good to the tenths of seconds-interpolate if in between tenths) and clicks it at moment of impact and then clicks it again as the ball crosses the net(fast clicking) take V=D/T
With a calculator in scientific notation input 39'/x seconds multiply by the factor 1.467 to get miles per hour or mi/hr.
39/.40 sec ~ 143 mi/hr
39/.50 sec ~ 111 mi/hr
39/.60 sec ~ 95 mi/hr
39/1.0 sec ~ 57 mi/hr
The factor 1 x 60 sec/ minute x 60 minutes/ hour= 3600 seconds = 1 hour
5280 feet = 1 mile
5280 ft/ 3600 sec = 1.467 miles/ hour
Credit were credit is due for this....