Spectra: What is that Supposed to Mean?
Okay, well, calcium is just an easily recognizable spectral feature, so I guess that's why they used it. The y-axis shows the relative intensity. I am assuming this is a spectrum that is normalized or scaled down so that the highest value of the intensity is 1, but this is not so important. The laboratory wavelengths of the absorption lines are shown in red in the graph. If you look carefully, you can see that the minimum of each spectral line does not exactly line up with these red tick marks. That is because the lines are Doppler shifted. M31 is moving and thus the light that we see has a different color (or frequency) from what we would observe in the lab. Let's focus on the left absorption line. The minimum is a bit to the left of the red mark. I conceive this to be approximately 4 angstrom off. The wavelength that you measure here on Earth will thus be 3630 angstrom and not 3934. If I take the formula from the previous page and rewrite I get
$$\frac{\Delta \lambda}{\lambda_0} = \frac{v}{c}$$
where $\Delta \lambda$ is the difference between the observed and the lab wavelength of the spectral line. In this case $\Delta \lambda = -4$ angstrom and $\lambda_0 = 3934$ angstrom. The speed of light is given in km/s, not in m/s, so the value is indeed correct. Now you can get the speed of M31 from
$$v = c\frac{\Delta \lambda}{\lambda_0} = 3 \times 10^5 \frac{4}{3934} = -305km/s$$
I entered this value and it was accepted.