Worker Efficiency An efficiency study showed that the average worker at Delphi Electronics assembled cordless telephones at the rate of
$$ f(t)=-\frac{3}{2} t^{2}+6 t+10 \quad(0 \leq t \leq 4) $$
phones/hr, $t$ hr after starting work during the morning shift. At what rate does the average worker assemble telephones 2 hr after starting work?