STEP 1 :

Equation of parabola : y^{2} = 16x

On comparing with : y^{2} = 4ax , 4a = 16 , a = 4

STEP 2 :

P(x , 8) lies on parabola y^{2 }= 16x & hence it must satisfy the equation of parabola

∴ 8^{2} = 16x

x = 4 , P(4,8)

STEP 3 :

Focal distance of point P(4,8) = PS

= PM ……….. FOCUS DIRECTRIX PROPERTY

= x + a

= 4 + 4

= 8 units