Football player on the 20 yard linme has to throw to receiver on the goal line 20 yards away. Quarterback must throw the ball over the outstreched arms or a defender on the 3 yard line and have it land in the receiver's hands. The quarterback is 6' tall. The defender can jump to stop a ball that is less than 9 feet off the ground. The receiver is 6' tall.

How high must the quarterback throw the ball to get it over the defender and still be catchable by his receiver?

How do you get the answer.

You can't answer that from the provided data.
The distance between defender and receiver on the moment of the ball being thrown is lacking, while it is of vital importance to calculate the required trajectory of the ball.