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

Given $|\vec{a}| = 4.5$ units and its direction is due north.

The magnitude of the vector $-4\vec{a}$ is $|-4\vec{a}| = |-4| |\vec{a}| = 4 \times 4.5 = 18$ units.

The negative sign indicates that the direction of $-4\vec{a}$ is opposite to the direction of $\vec{a}$.

Since $\vec{a}$ points due north, $-4\vec{a}$ points due south.

Thus, $-4\vec{a}$ is a vector of magnitude $18$ units pointing due south.