Galaxies are sprawling systems of dust, gas, dark matter, and anywhere from a million to a trillion stars that are held together by gravity. Nearly all large galaxies are thought to also contain supermassive black holes at their centers.
Answer:

Explanation:
Change in velocity considering the x component will be
Final velocity-Initial velocity

Change in velocity considering the y component will be
Final velocity-Initial velocity

Resultant change in velocity
Acceleration= change in velocity per unit time hence

I am pretty sure the answer to your question is B
-- Equations #2 and #6 are both the same equation,
and are both correct.
-- If you divide each side by 'wavelength', you get Equation #4,
which is also correct.
-- If you divide each side by 'frequency', you get Equation #3,
which is also correct.
With some work, you can rearrange this one and use it to calculate
frequency.
Summary:
-- Equations #2, #3, #4, and #6 are all correct statements,
and can be used to find frequency.
-- Equations #1 and #5 are incorrect statements.