When a potential difference of 12 v is applied to a wire 6.8 m long and 0.35 mm in diameter the result is an electric current of 2.1

a?

The total resistance is R = voltage / current

This resistance R = (L/A) r where:

r is the resistivity

L is the length of the conductor

A is the cross-sectional area of the conductor.

thus r = RA/L

You are given L; you can compute R from the voltage and current you are given; and the cross-sectional area of a round wire is (pi) (radius^2) or (pi/4) (diameter^2)