Step 1: Establish a rectangular coordinate system with point C as the origin of the coordinate axis, and calculate the coordinates of point AB as (-3,0) (4,0) respectively.

Step 2: Find the coordinates of point P and point Q and the midpoint coordinates of PQ with unknown numbers, and then find the linear equation of PQ.

Then find the DE linear equation.

Step 3: Establish an equivalent equation according to the relationship between two vertical straight lines, and find out the unknowns that satisfy the meaning of the question.

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