In Problem 29, the width of the strip is $3.5 \mathrm{~cm}$, the magnetic field is $0.43 \mathrm{~T}$, the Hall voltage is measured to be $7.2 \mu \mathrm{V}$, the thickness of the strip is $0.24 \mathrm{~mm}$, and the current in the wire is $54 \mathrm{~A}$. What is the density of carriers (number of carriers per unit volume) in the strip?