For the chocolate the in the microwave,
The calculation of the frequency is using
F=speed of light/wavelength
I put nodes for the three dots of MELTED chocolate which is wrong it is ANTINODES.
But even if you got that wrong the wavelength needed to calculate the frequency will still be the same.
Distance between a node and a node is 6cm
Distance between an antinode and antinode is 6cm. They are both half a wavelength anyway??
So 0.12m is the wavelength.
Gets you 2.5x10^9 Hz.