A patient receives a 150-mg injection of a drug every 4 hours. The graph shows the amount $f(t)$ of the drug in the bloodstream after $t$ hours. Find
(a) $\displaystyle \lim_{t \to 12^-} f(t)$
(b) $\displaystyle \lim_{t \to 12^+} f(t)$
(What's the significance of these one-sided limits?)

(a)
(b)