The numbers must be split into a 3 digit number and a 2 digit number. Since the 3 digit number is double the 2 digit number, it must be divisible by 201 (for example 13869 = 69 x 201). This can be easier seen doing multiplaction by hand.
Since 201 = 67 x 3, and the two digit number times 201 or (67 x 3) times 3 is a perfect square, the following equation is true: ((? x (67 x 3)) x 3) = perfect square.
Since 67 needs to be accounted for, it must be some perfect square times 67 for the two digit number. The only one that works here is 67 itself. So the 5 digit number is 13467.
134 = 67 x 2, and 13467 x 3 = 201^2
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