The volume of a box is the length times the width times the height. The box you want has its length, width and height equal so if you call the length of the box x feet then the volume is x x x or x3 cubic feet. Hence you want to fid x so that
x3 = 1.26 cubic feet.
Your calculator may be able to calculate x for you by taking the cube root of 1.26 but you can also use the guess-and-check method.
Since x3 = 1.26 I know that x is larger than 1 so try x = 1.1.
1.13 = 1.331 so 1.1 is too large.
Try x = 1.09
1.093 = 1.295 so 1.09 is also too large.
Try another value.
Stephen and Penny