Write the Word or Phrase That Best Completes Each Statement $$\mathrm { D } = 8 \mathrm { e } ^ { - 0.6 \mathrm {~h} }$$

Write the word or phrase that best completes each statement or answers the question.
-The formula $$\mathrm { D } = 8 \mathrm { e } ^ { - 0.6 \mathrm {~h} }$$ can be used to find the number of milligrams D of a certain drug that is in a patient's bloodstream h hours after the drug
has been administered. The drug is to be administered again when the amount in the bloodstream reaches 4 milligrams.
What is the time between injections?