you are given of the following data:
l = 24cm
w = 8 cm
h = 6cm
and that the formula is d = √(l²+w²+h²)
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getting the value of d you'll have
d = √(24² + 8² +6²)
d = √(576 + 64 + 36)
d = √(676)
d = 26cm
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therefore the length of the diagonal is 26cm.