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)

The answer to this is decibel