Answer
$y=900(0.91)^t$.
Work Step by Step
Initial value $a=\$900$
Rate of decay $r=9\%=0.09$
The exponential decay function is
$\Rightarrow y=a(1-r)^t$
Substitute $900$ for $a$ and $0.09$ for $r$.
$\Rightarrow y=900(1-0.09)^t$
Simplify.
$\Rightarrow y=900(0.91)^t$.
