Answer
(a) $m(0)=13kg$
(b) $m(45)\approx6.62kg$
Work Step by Step
So, we have a function of $m(t)=13e^{-0.015t}$, where $t$ stands for days passed and $m(t)$ the amount of mass ($kilograms$) remaining after $t$ days.
(a) The mass when $t=0$ (Which means that the substance hasn't started decaying yet) is:
$m(0)=13e^0=13\times1=13 kg$
(b) When $t=45$
$m(45)=13e^{-0.015\times45}=13e^{-0.675}\approx6.62kg$
