Step 1: Basic Principle
Projections on coordinate axes are the components of the vector.
Step 2: Solution Procedure:
If $l, m, n$ are the projections (direction cosines times magnitude), then the vector $\overrightarrow{PQ} = 12\hat{i} + 3\hat{j} + 4\hat{k}$.
Magnitude $|\overrightarrow{PQ}| = \sqrt{12^2 + 3^2 + 4^2} = \sqrt{144 + 9 + 16} = \sqrt{169} = 13$.
Step 3: Required Answer:
The magnitude is $13$.