Solving Simple Harmonic Motion: Cart Position and Time Comparison

[asz304]

Homework Statement

The position of a an air-track cart that is oscillating on a spring is given by (12.5cm)cos[(18.0s-1)t].
At what value t after t=0 is the cart first located at x=12.2 cm?

The Attempt at a Solution

12.2 cm = (12.5cm)cos[(18.0s^-1)t].

How do I factor t out?

Thanks

 

[zorro]

What is s?

 

[asz304]

I believe s^-1 is f which is equivalent to 1/T. T is period which is not equal to t time. Anyone could help me?