I have my solution and my doubt in the attachment that followed.
Your solution is right. But the equation of motion might be easier to use if you chose the sin instead of cos:
[tex] y = 3sin(\pi t - \pi/2)[/tex]
This is because at t = 0, the mass is at maximum negative amplitude ([itex]\theta = -\pi/2[/itex])
[tex]d = y-y_0 = 3sin(\pi t - \pi/2) - 3sin(-\pi/2) = 3sin(\pi t - \pi/2) + 3[/tex]
For 1), where t = 1 sec., d = 3 +3 = 6
For 2), where t = .75 sec,[itex]d = 3sin(\pi/4) + 3 = 5.12[/itex]
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