This question is soo hard. Could someone please help me with the answer?

So you're seeking P(t) < 35, or 75e^(-.0015t) < 35. Then we have e^(-.0015t) < 35/75. Take logs of both sides: -.0015t < ln(35/75) ==> -.0015t < -.762, and t > 508 (rounded). So answer e.