Answer
Using the data given, 80ml/kgx90kg=7200ml=7.2l=volume pumped in 1 min normally.
Renal flow=21% of cardiac output=0.21x7.2l/min=1.5l/min
Work Step by Step
This is a simple matter of watching units and multiplying relevant figures. One hint: you have done things correctly if you end up with the correct final units.