Problem about Nyquest signalling theorem?
I know the power of the signal, but how to get the noise power?


The quantization noise can be approximated as a sawtooth wave that has a peak-to-peak amplitude equal to the step size of the quantizer. You just need to calculate the RMS voltage of this sawtooth wave and compare it to the RMS value of the sinewave signal.

BTW, this part of the problem really has nothing to do with Nyquist, so your title is a bit misleading.