Answer
$x\approx8.17$
Work Step by Step
We can set the cross products of the given proportion equal to each other and then solve for the unknown variable.
$7\times21=x\times18$
$147=18x$
Divide both sides by 18.
$\frac{147}{18}=\frac{18x}{18}$
$x\approx8.17$
