AoPS Wiki:Problem of the Day/June 18, 2011

In trapezoid $ABCD$ , $m\angle A=m\angle D=90^{\circ}$ . Base $AB$ has a length of $9$ while base $CD$ has a length of $15$ . Diagonals $AC$ and $BD$ meet at point $X$ . If the perimeter of the trapezoid is $42$ , find the distance from $X$ to $CD$ .