Concept:
Parametric form of parabola
Step 1: Vertex = (0,0), point $B = (at^2, 2at)$.
Step 2: Slope of $AB$:
\[
\frac{2at}{at^2} = \frac{2}{t}
\]
Step 3: Perpendicular slope = $-\frac{t}{2}$.
Step 4: Equation of $BC$ through $B$ and meet x-axis.
Step 5: Projection on axis simplifies to:
\[
4a
\]
Conclusion:
Answer = $4a$