Asuitcase measures 24 inches long and 18 inches high. what is the diagonal length of the suitcase to the nearest tenth of a foot?

Diagonal
draw an immaginary line diagonal
now yo has right triangle
 
pythagorean theorem
if yo has a right triangle with legs legnth a and b and hypotonuse c (diagonal), then a^2+b^2=c^2

legs are 24 and 18
18^2+24^2=c^2
324+576=c^2
900=c^2
sqrt both sides
30=c

diagonal legnth=30 inches

Whose mistake? can you give more details to what the question is asking?