#### Answer

$x = 6$

#### Work Step by Step

To solve a proportion, we use the cross products property, multiplying the numerator of one ratio with the denominator of the other ratio, and vice versa:
$2(21 - x) = 5x$
Distribute on the left side of the equation:
$42 - 2x = 5x$
Add $2x$ to both sides of the equation to collect variable terms on the right side of the equation:
$42 = 7x$
Divide each side of the equation by $7$ to solve for $x$:
$x = 6$