Answer
This model projects in $2013$ when $1170.9$ million smartphones will be sold.
Work Step by Step
$S(t) = 21.4(1.56)^{t}$
$1170.9 = 21.4(1.56)^{t}$
$\frac{1170.9}{21.4} = (1.56)^{t}$
$t = \log_{1.56} \frac{1170.9}{21.4}$
by GDC / calculator
$t = 9$
$2004 + 9 = 2013$