Pendulum completes swing (back and forth) each 6s. As it swings, the distance, d, in cm, depends on time t, in seconds. At t=1.3 d is at its max of 110cm. Its min is 50cm. D is sinusoidal function of t.

I got the equation y=80+60cospi/3(t-1.3) using the methods from the book (find the sine axis, amplitude, high point and B), is there a better way/ faster way to do it?

The second question asks for the fastest pendulum swing, and where it is swinging its fastest. I don't understand how to solve for this question.

For the third question what is the first positive value of t at which the pendulum is swinging 0cm/s and where is the pendulum at this time- do you find the third derivative y'' and then solve for y''=0? then put the value t into the main equation y?