[anita-573]

Hi, any help would be greatly appreciated as I have no idea how to begin, the answer is meant to be 1.27 mg mL-1.

The concentration of iron in a blood sample was determined by
flame atomic absorption spectrophotometry as follows: 2 mL of blood was
extracted with 20 mL of methanol and filtered. 1 mL of the extract was diluted to
20 mL with methanol and aspirated onto the flame. The absorbance was 0.115
(arbitrary units). A 10 mg/L standard iron solution yielded a signal of 0.180 under
the same conditions. Calculate the quantity of iron in the sample in mg mL-1

[Borek]

Hint: how does the signal depend on the iron concentration in the solution that is being feed into the flame?

[anita-573]

so the change in the signal equals the change in iron concentration.

[Borek]

so the change in the signal equals the change in iron concentration.

No, it doesn't "equal" the change. It reflects the change.