If the omnibus analysis of variance for a complex design reveals a statistically significant interaction effect, the source of the interaction effect may be identified using

36 millimeters

Step-by-step explanation:

The negative produce a 4" by 6" photograph

So, the width of the photo = 4"

And the length of the photo = 6"

if the width is 24 millimeters, let the length = x

So, width : length

         4     :     6

        24   :      x

Using the ratio and the proportional

∴ x = 6 * 24/4 = 36 millimeters

So, the length of each negative = 36 millimeters