Question

what is the width and length of a box that is 315 cu. units?

Answer 1

hard to say. If you want integer units, you could factor 315.

315 = 3*3*5*7

so, possible dimensions are

9x5x7
3x15x7
3x5x21
and, of course,
1x1x315
1x3x105
1x5x63
1x7x45
1x9x35
1x15x21