What do triangles mean in frequency aliasing depiction?
The sampling rate is 6 kHz, so all the spectral content of what you are sampling will appear in the frequency range of -3 kHz &lt; f &lt; 3 kHz. The unshaded triangle from 3 kHz to 9 kHz is where the actual spectral content is.

When you sample at 6 kHz the spectral content of the unshaded triangle will appear as the triangle in the shaded region. More specifically the frequency at 4 kHz will appear at -2kHz, and the frequency at 7 kHz will appear at 1 kHz. Another way of saying this is, if you sample at 7kHz tone at 6 kHz it looks exactly the same as a 1 kHz tone sampled at 6 kHz. That's why aliasing leads to ambiguity - you can't tell whether that 1 kHz tone actually came from a 1 kHz, or a 7 kHz tone.

So the unshaded triangle represents where the original spectral content is, and the triangle in the shaded portion is where that triangle gets mapped to (when sampling at 6 kHz).