Pendulum equation

[CastleBravo]

I am working on this pendulum project for my calculus class. Right now I am stuck between a rock and this hard place as I don't really know how to accomplish Tasks 3 and 5. Any help would really be appreciated. Thanks

 

[HallsofIvy]

What have you tried? You can differentiate that given function, can't you? You just put the given function into the equation and show that it satisfies the equation.