Let a be a vector of magnitude 4.5 unit pointing due north....

Given that the magnitude of vector $\vec{a}$ is $|\vec{a}| = 4.5$ units and its direction is due north.

When a vector is multiplied by a positive scalar, its magnitude is multiplied by the scalar and its direction remains unchanged.

The magnitude of the new vector $3\vec{a}$ is $3 \times |\vec{a}| = 3 \times 4.5 = 13.5$ units.

Since $3$ is a positive number, the direction of $3\vec{a}$ is the same as $\vec{a}$, which is due north.

Thus, $3\vec{a}$ is $13.5$ units due north.