Why is - 2x - 8 an algebraic expression

Because it contains an un-known variable denoted as X

Because algebra means finding the unknown

it is usually used in real life

for example if if 1/2 of the legnth of a line is 8 then what is the legnth

so then 1/2 x=8 and that is algebraic

since it has x, or an unknown in it, it is an algebraic expression