Geometry: Common Core (15th Edition)

$x = 13$
In a parallelogram, opposite angles are congruent. Let us set opposite angles equal to one another to solve for $x$: $x + 38 = 4x - 1$ Subtract $38$ from each side of the equation to isolate constants on one side of the equation: $x = 4x - 39$ Subtract $4x$ from each side of the equation to isolate the variable on one side of the equation: $-3x = -39$ Divide each side of the equation by $-3$ to solve for $x$: $x = 13$