You can use fourier transform to obtain frequency representation of a signal from not only time domain but also spatial domain. most famous kind of signal which lives in a spatial domain is a single image. an ordinary rgb image is a 2-dimensional (2d) spatial signal. in order to get frequency representation of a signal which lives in a spatial domain you have to use spatially encoded information. in this ques- tion you are required to give derivation of 2d fourier transform, starting f fourier series representation of periodic signal, for continuous values most probably you have an intuition about what the different frequencies repre sent for one-dimensional signal. try to extend the intuition for 2d spatial signal and explain what the different frequencies in frequency domain correspond to in spatial domain.