I'm sorry, do you have any more detail. i'm not sure what this means?
X=18 (3/8)
(8x/7) +9 = 30
subtract 9 from both sides
8x/7=21
multiply 7 from both sides
8x=147
divide 8 from both sides and you get
x=18 (3/8)
It is more than just a quadrilateral. In fact it is going to be hard to pick.
These facts suit a square, a rectangle, a rhombus, and a parallelogram. And the above statement is true, but maybe a little harder to prove than the converse of the statement, which is the usual one you find.
The converse is "If you have a parallelagram, the diagonals bisect each other."
You might think a trapezoid deserves some mention. The diagonals of a trapezoid do not bisect each other.
