Answer
$x = 2$
Work Step by Step
A rhombus has four congruent sides, so we can set the expressions for the two sides equal to one another to find $x$:
$8x - 1 = 3x + 9$
Subtract $3x$ from each side of the equation to move the variables to the left side of the equation:
$5x - 1 = 9$
Add $1$ to each side of the equation to move constants to the right side of the equation:
$5x = 10$
Divide each side of the equation by $5$ to solve for $x$:
$x = 2$
