Answer
(a) $C/L = 120 \times 10^{-12} \,\text{F/m}$
(b) $Q = 118 \times 10^{-12} \,\text{C} $
Work Step by Step
(a) The capacitance per unit length $C/L$ for a cylindrical shape is given by
$$C/L = \dfrac{2\pi \epsilon_o}{\ln(r_b/r_a)} $$
Substitute to get $C$
\begin{align}
C/L &= \dfrac{2\pi \epsilon_o}{\ln(r_b/r_a)} \\
&= \dfrac{2\pi (8.85 \times 10^{-12} \,\text{F/m})}{\ln(3.5 \,\text{mm}/2.2 \,\text{mm})}\\
&= 120 \times 10^{-12} \,\text{F/m}
\end{align}
(b) The charges on the plates are given by
$$Q = CV$$
Where $C = 120 \times 10^{-12} \,\text{F/m} (L) $. Let us substitute the values $C$ and $V$ to get the charge on each plate
\begin{align*}
Q &= CV \\
&= 120 \times 10^{-12} \,\text{F/m} (L) V\\
&= 120 \times 10^{-12} \,\text{F/m} (2.8 \,\text{m}) (350 \times 10^{-3} \,\text{V})\\
&= 118 \times 10^{-12} \,\text{C}
\end{align*}
