Op-amp problem....

I searched three detailed books that had hundreds of pages about
op-amps and I can't find the solution to this problem. I think there's
a simple solution but I just can't work it out..
Here's the schematic of the op-amp differential amplifier:
:
R2
---------/\/\/\/\/\----------
| |
| |
| |
| \ |
R1 | \ |
Vi_1 --------------/\/\/\/\/\--------- - \ |
| ----------------------|
V_o
R3 | /
Vi_2 --------------/\/\/\/\/\---------- + /
| | /
|
| R4
\-----/\/\/\/\/\-----
|
|
Gnd
The first part is to give V_o in terms of the other variables, which is
easy enough and is in the books. The next part is:
Vi_1(t) = 10^(-3) Sin(2pi x 10^3t) + Vn(t) Vi_2(t) = 10^(-3)
Cos(2pi x 10^3t) + Vn(t)
R_1 = R_2 = 1kohm R_2 = R_4 =
30kohm
where Vn(t) denotes unwanted noise. If the Common Mode Rejection Ratio
(CMMR) of the op-amp is 97 dB and both inputs are "contaminated" with
50 Hz noise of average amplitude 1V, calculate the ratio of output
signal amplitude to average output noise amplitude.
I know what the CMMR equation is but the frequencies are confusing
me...