Answer:
29.7 inches
Step-by-step explanation:
if it's a square box we can assume that all sides are equal. so the width is also equal to 21 inches.
to find the diagonal we use the pythagorean theorem which states that a^2 +b^2 =c^2 where c is the diagonal and (a) and (b) are 21
so diagonal is the root of
21^2 + 21^2
answer is 29.7
