Question

Meagan is sitting in a rocking chair, the distance d(t-) between the wall and the rear of the chair varies sinusoidally with time t. At t = 1s , the chair is closest to the wall and d(1) = 18 cm. At t = 1.75s the chair is farthest from the wall and d(1.75 ) = 34m

how far is the chair when no one is rocking it?

and

what is the distance between wall and the chair at t= 8s?