A wire of length 15.0 metre and diameter 6.0 mm extended by 0.5 mm when a force of 400 Newton was used to stretch it from its end calculate the stress on the wire.
The amount of stretch is STRAIN not stress, and is irrelevant here.
The STRESS is load divided by cross section area.
diameter = 0.006 meters
so radius = 0.003 meters
stress = 400 Newtons / [pi(0.003)^2 meters^2]