If PSQ is a focal chord of the parabola y= 8x such that SP= 6, then the length of SP is

Since the semi-latus rectum of a parabola is the harmonic mean
between the segments of any focal chord of the parabola, SP,4, 
SO are in HP. Therefore, 

$\begin{array}{l}4=\frac{2\mathrm{SP}\times \mathrm{SQ}}{\mathrm{SP}+\mathrm{SQ}}\\ \mathrm{or} 4=\frac{2\left(6\right)\left(\mathrm{SQ}\right)}{6+\mathrm{SQ}}\\ \mathrm{or} 24+4\left(\mathrm{SQ}\right)=12\left(\mathrm{SQ}\right)\\ \mathrm{or} \mathrm{SQ}=3\end{array}$