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(a) 28 x 63 = (2 x 2 x 7) x (3 x 3 x 7);

This you can rearrange to give (2 x 3 x 7) x (2 x 3 x 7) = 42 x 42 (perfect square).

(b) similarly if 28k is a perfect cube, then 2 x 2 x 7 x k is perfect. So k must be 2 x 7 x 7 to make sure all the factors can cube, and 28k = (2 x 7) x (2 x 7) x (2 x 7).

(Yes, your answer to (b) is wrong, sorry; k = 98.)

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