L=8 w=4 h=2

Find the diagonal (d) of the base

Length = 8

width = 4

height = 2

The question is which base do you mean because there are 3 different bases. I assume you mean largest one which in this case is 8x4 base

Diagonal of it is:

d = sqrt (8^2 + 4^2) = sqrt (64+16) = sqrt (80) = sqrt (16*5) = 4*√5