As given in the problem that given quadrilateral RSTU is a parallelogram .
And it is a property of a parallelogram , that the diagonals bisects each other .
It means point V is the mid-point of US .
And it implies UV=VS
So we get,
x-3 = 3x-13
Now we have to solve for x
subtract 3x from both sides
x-3x -3 =3x-3x -13
-2x -3 = -13
add 3 to both sides
-2x -3+3 = -13 +3
-2x = -10
Divide both sides by -2
x=5