Link Power Budget Problem

Would anyone be able to assist me with this....homework problem?
"Consider a channel whose transmitter noise = its receiver noise 1microWatt. In addition, let channel noise accumlate at 50nW/km.
Ignore its attenuation. Let the signal's transmit power = 100mW and let the signal attenuate at -6dB/km. If we require the receiver's SNR dB, what is the maximum line length?"
I'm not quite sure how to solve this. I know the SNR is the ratio of the power of the signal at the receiver to the noise at the receiver. But how exactly do I incorporate the noise into this equation given that the units for noise are not in dB? Any help is appreciated. Thanks.
Vincent
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Vincent wrote:

Is it too late to withdraw from the course and get your tuition back?
Bill
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Ha ha ha....that's a good one. I wish!
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