Answer
$f(x)=4.5\left(9^{x}\right)$
Work Step by Step
Exponential models have form: $f(x)=y=Ab^{x}$
When $x$ increases by one unit, the value of $f$ is multiplied by $b.$
$A=f(0)$
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If x increases one whole unit, (twice by half-unit)
f will increase by $3\cdot 3=9=b.$
$A=f(0)=4.5$ (given) so
$f(x)=Ab^{x}=4.5\left(9^{x}\right)$
