$\theta = \dfrac{TL}{JG}$
$3^\circ \left( \dfrac{\pi}{180^\circ} \right) = \dfrac{12(6)(1000^3)}{\frac{1}{32}\pi d^4 (83\,000)}$
$d = 113.98 \, \text{mm} \,$ answer
$\tau_{max} = \dfrac{16T}{\pi d^3} = \dfrac{16(12)(1000^2)}{\pi (113.98^3)}$
$\tau_{max} = 41.27 \, \text{MPa}$ answer