Try this option:
1. note, that the number 84 is divided by numbers: 1;2;3;4;6;7;12;14;21;28;42 and 84. It means, that measurements of the length may be: 1;2;3;4;6;7;12;14;21;28;42;84 and for the width: 84;42;28;21;14;12;7;6;4;3;2;1.
2. according the numbers calculated in item 1, the possible number measurements for the length and width of the sandbox are (length; width):
1;84
2;42
3;28
4;21
6;14
7;12
12;7
14;6
21;4
28;3
42;2
84;1
