An 8 kg mass is supported by an aluminum wire of length 80 cm abd diameter 2.0mm. How much will the wire stretch?

First find from the book or Internet the Young's modulus (E) for aluminium.

I use E=69 GPa for aluminium.

Using stress = force (mg) / cross sectional area

ε=Strain = extension/unit length
= stress/E

stretching = extension
=ε * length
=stress*length/E

you can take it from here.