$\"\"$

The function is $\"\"$ .

Where $\"\"$ represents the number of hours.

$\"\"$ represents the number of milligrams.

(a)

Find the time between injections.

The drug is to be administered again when the number of milligrams reaches $\"\"$ .

Therefore, $\"\"$ .

Substitute $\"\"$ in $\"\"$ .

$\"\"$

From the definition $\"x$ if and only if $\"y$ .

$\"\"$

The time between the injections is $\"\"$ hours.

$\"\"$

The time between the injections is $\"\"$ hours.