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Answer:
 a) P(t) = 6.29e^(0.0241t)
 b) P(6) ≈ 7.3 million
 c) 10 years
 d) 28.8 years
Step-by-step explanation:
a) You have written the equation.
 P(t) = 6.29·e^(0.0241·t)
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b) 2018 is 6 years after 2012.
 P(6) = 6.29·e^(0.0241·6) ≈ 7.2686 ≈ 7.3 . . . million
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c) We want t for ...
 8 = 6.29·e^(0.0241t)
 ln(8/6.29) = 0.0241t
 t = ln(8/6.29)/0.0241 ≈ 9.978 ≈ 10.0 . . . years
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d) Along the same lines as the calculation in part (c), doubling time is ...
 t = ln(2)/0.0241 ≈ 28.7613 ≈ 28.8 . . . years