Concept:
The distance of a point \( (x, y, z) \) from the \( x \)-axis is determined by the length of the perpendicular dropped to that axis. On the \( x \)-axis, the \( y \) and \( z \) coordinates are zero. The projection of \( (x, y, z) \) on the \( x \)-axis is \( (x, 0, 0) \).
Step 1: Identify the projection on the \( x \)-axis.
Point \( P = (3, 12, 5) \).
The point on the \( x \)-axis closest to \( P \) is \( A = (3, 0, 0) \).
Step 2: Use the distance formula.
\[ \text{Distance} = \sqrt{(3 - 3)^2 + (12 - 0)^2 + (5 - 0)^2} \]
\[ \text{Distance} = \sqrt{0^2 + 12^2 + 5^2} \]
\[ \text{Distance} = \sqrt{144 + 25} = \sqrt{169} = 13 \]