Hi Ginger,

There is not enough information in what you sent us to determine the lengths of the diagonals. I am going to illustrate this with some diagrams.

Draw a line $PQ$ of length 40 cm and then draw a semicircle with center $Q$ and length 15 cm. (My diagram is not to scale.)

Choose a point $G$ on the semicircle, draw the line segment $QG$ and then add two line segments to complete a parallelogram. This parallelogram has sides of length 40 cm and 15 cm.

Now choose a different point $B$ on the semicircle, draw the line segment $QB$ and then add two line segments to complete a parallelogram. This parallelogram also has sides of length 40 cm and 15 cm.

Finally look at the two parallelograms together.

Clearly the diagonals $PG$ and $PB$ are of different lengths.

Penny