$V_{max} = 600(12) = 7200 ~ \text{N}$
$M_{max} = 600(12)(6) = 43,200 ~ \text{N}\cdot\text{m}$
$I = \dfrac{\pi}{64}(273^4 - 255^4) = 65,105,640.45 ~ \text{mm}^4$
Part (1)
$Q = \frac{1}{2}\pi (136.5^2) \cdot \dfrac{4(136.5)}{3\pi} - \frac{1}{2}\pi (127.5^2) \cdot \dfrac{4(127.5)}{3\pi}$
$Q = 313,753.5 ~ \text{mm}^3$
$f_v = \dfrac{VQ}{Ib} = \dfrac{7200(313,753.5)}{65,105,640.45(18)}$
$f_v = 1.928 ~ \text{MPa}$ ← answer
Part (2)
$f_b = \dfrac{Mc}{I} = \dfrac{43,200(1000)(136.5)}{65,105,640.45}$
$f_b = 90.57 ~ \text{MPa}$ ← answer
Part (3)
$\delta_{\text{due to force } P} = \delta_{\text{due to uniform load}}$
$\dfrac{PL^3}{3EI} = \dfrac{wL^4}{8EI}$
$\dfrac{P}{3} = \dfrac{600(12)}{8}$
$P = 2700 ~ \text{N}$ ← answer
