Graphical Analysis of damped oscillator

[LCSphysicist]

Homework Statement

The question is to find Q, the factor Q, of the system.

Relevant Equations

All below,
Q = w/y

First of all, i tried to find w, the angular frequency, by calculating the oscillations from ta to tc, there is ~ 20 oscillations coursed.
so,
w = 2*pi*20/(tc-ta)
ta = 0, tc = 0 + 5.2 ms

And tried to find the factor gama y by A(t) = A*cos(Φ + wt)*e^(-yt/2)

A(0) = 2.75u = A*cos(Φ)
1u = A*cos(Φ + 20pi)*e^(-y*5.2/2)

So y ~ 2*ln(2.75)/5.2*10-³

w ~ 2*pi*20/5,2*10-³

Q = w/y ~ 64

Is this right?

OBS: yes, i forget write 10^-3 in the equations, that's ok, since it will cut in the division.

 

[haruspex]

I get a slightly smaller answer, but the method looks right to me.