Hard A-LEVEL Biology question: HELP!

Hii... I'm really struggling with this A-level biology question. I've really tried to get to the answer but I'm just not getting it.

Here's the graph and the information required:
After the 3rd injection, the concentration of anti-meningitis antibody in the blood was also measured after 60 days, after 120 days and then after 180 days. Figure 4 shows the scientists’ results. Each plotted point in Figure 4 is the result for
a different mouse
The scientists discovered that the concentration of anti-meningitis antibody of the
mouse labelled Z in Figure 4 decreased after the 3rd injection at a constant rate of 0.027 arbitrary units per day.
Use this information and Figure 4 to calculate the number of days after the 3rd injection the antibody concentration is higher than the protective antibody concentration for this mouse (2 marks).

I would really appreciate the help!

Alright so if you look at figure 4 you can see that the antibody concentration for mouse Z is 5.1 and that the protective antibody concentration is 2.1

The question asks how many days is the concentration of antibodies in mouse Z is higher than the protective concentration, and you also know the rate of decrease of the concentration.

So all you would do is take 5.1 from 2.1 to get 3, which is how much more concentration is in mouse Z than the protective antibody concentration.
And then divide 3 by 0.027, as this gives the amount of day for the concentration of antibodies in mouse Z to reach the protective antibody concentration, giving an answer of 111 (3sf)

Sorry if I explained poorly and let me know if you need any more help.