$0\mathrm{.}84-$m$-$long copper wire has a resistance of $1\mathrm{.}21.$ What is the diameter of the wire? The esistivity of copper is $=1\mathrm{.}70{10}^{-8}*m.$

inter your answer in units of millimeters. For example, if your calculation produces a value of $\mathrm{.}000123$ m $,$ first convert units to mm as $0\mathrm{.}123$ mm $,$ then enter your answer as $0\mathrm{.}123.$