#### Answer

$x=1.63\times10^{19}$

#### Work Step by Step

Add $(7.2\times10^{18})$ to both sides.
$x=9.1\times10^{18}+7.2\times10^{18}$
$x=(9.1+7.2)\times10^{18}$
$x=16.3\times10^{18}$
Write 16.3 in scientific notation.
$16.3=1.63\times10^{1}$
$x=1.63\times10^{1}\times10^{18}=1.63\times10^{19}$