(a)
Since line is parallel to the plane vector, $2\overrightarrow{i} + 3\overrightarrow{j} + \lambda\overrightarrow{k}$ is perpendicular to the normal to the plane
$2\overrightarrow{i} + 3\overrightarrow{j} + 4\overrightarrow{k}$
$\Rightarrow 2 \times 2 + 3 \times 3 + 4\lambda = 0$
$\Rightarrow \ \lambda = - \frac{13}{4}$