Regular exercise can help a person lower their resting heart rate. Suppose an individual’s resting heart rate, in beats per minute, is modeled by the function f(x) = –0.1x 3 + 0.6x 2 + 85 after x weeks of regular exercise.

How many weeks of exercise are necessary to lower the individual’s resting heart rate below 70 beats per minute?

7 weeks
8 weeks
9 weeks
10 weeks