When you take a look at parabolas, we know that their maximum or minimum values are usually on the axis of symmetry. This axis of symmetry has an equation of
x = -b/2a,
In this question, the axis of symmetry is
x = -600 / -2
x = 300
This means that planting 300 trees will maximize your yield. If you substitute for x = 300 into the equation, the maximum yield will be
Try this option: 1 meter=1000 mm; 1 hour=60 min. From another side: 1 mm=0.001 m. and 1 min=1/60 h. Using this rule: 620 mm/h=0.06*620 m/h=37.2 meters per hour.