if the answer to the cubic inches of a rectangle is 258 3/4 how do i arrive at this answer

Rectangles are 'planar' figuresn not solids....so they do not have cubic inches as a measurement....they have SQUARE INCHES as an area measurement  ( or square cm    etc)         you cannot tell anything more about this rectangle ....there are infinite possibilities

area = length  X  width       so you will need either the length or the width  along wi th the area to compute the other measurement.