Robin throws a baseball 90.0 miles per hour and bluebird throws a baseball 121 feet per second which pitcher throw a baseball faster

To get who is the fastest between Robin and Bluebird we need to write the speed in the same units, either miles per hour or feet per second. Writing Robin's speed in feet per second we shall have:
1 mile=5280 ft
thus 90 mph in ft/sec will be:
(90*5280)/3600
=132 ft/sec
given that:
132ft/sec<121ft/sec, we conclude that Robin is faster than Bluebird