Spherical Wave - What is the phase?

[samee]

Homework Statement

A spherical wave with a wavelength of 2.0 m is emitted from the origin. At one instant of time, the phase at r=4.0m is pi radians. At that instant, what is the phase at r=3.5m?

Homework Equations

D(x,y,z,t)=Asin(kr-wt+phi)
k=(2pi)/(lambda)
w=(2pi)/T

The Attempt at a Solution

(kr-wt+(phi))=phase constant
D(x,y,z)=displacement, or here, radius
A cancels and doesn't matter.
4/(pi)=3.5/x
x=3.5(pi)/4 <---- which is wrong.

So then I tried subtracting the equations...
4=Asin(pi)
-3.5=-Asinx
.5=Asin(pi)-Asinx
.5=Asinx <---- Then I didn't know what to do with it. Help please?

 

[samee]

Nevermind . . .

 

[Ben Winter]

I'd sure like to know.