What does phase of the motion in terms of cosine displacement mean?

[Maskawisewin]

What does "phase of the motion in terms of cosine displacement" mean?

I'm getting tripped up on the wording of this homework question.

Homework Statement

Measured acceleration: An accelerometer has measured the simple harmonic motion shown in the image below.

Homework Equations

You want to describe the position as a function of time as a cosine: $x(t)=A\cos(\omega t-\phi)$.
What is the phase of the motion shown in the graph (in terms of a cosine displacement)?

The Attempt at a Solution

I've tried reading my textbook for "phase of the motion". There's a section that talks about a rotating vector phasor that references the unit circle. Is this the right path? Are they talking about what the cosine is calculating (i.e. what's inside the brackets of the cosine)?

 

[frogjg2003]

The phase of a periodic function is the displacement the function has from some predefined 0 point. For example, sin(x) is 90° out of phase from cos(x). The question is just asking for $\phi$ in $x(t) = A\cos(\omega t+\phi)$.

 

[Maskawisewin]

Thank you.