I think I might have got it....
So it looks like 1.5^2 + b^2 = 21^2.
So 441 - 2.25 = 438.75, the square root of which is 20.9464.
Seems to get it....
Start X0 Y-1.5 Z0
End X0 Y1.5 Z0
Center X-20.9464 Y0 Z0
Arc Length 3.0026
Plane G17 - XY
Start Angle 355.904
End Angle 4.096
GCode G03 X0 Y1.5 I-20.9464 J1.5