Geometry: Common Core (15th Edition)

$x = 5$
Theorem 6-6 states that in a quadrilateral that is a parallelogram, its diagonals bisect one another. This means that each segment of the bisected diagonal is equal. We can set the two segments of the bisected diagonals equal to one another: $3x = 4x - 5$ $-x = - 5$ Divide both sides by $-1$ to solve for $x$: $x = 5$