Answer is: 1130 microliters must be administered.
activity(selenium-75) = 2.3 mCi/mL.
radioactivity(Se-75) = 2.6 mCi; mCi is millicurie.
The curie (symbol: Ci) is a non-SI unit of radioactivity.
V(Se-75) = radioactivity(Se-75) ÷ activity(Se-75).
V(Se-75) = 2.6 mCi ÷ 2.3 mCi/mL.
V(Se-75) = 1.13 mL; volume.
V(Se-75) = 1.13 mL · 1000µL/mL.
V(Se-75) = 1130 µL.