Answer:
1/4πε₀[Hx/ (√x² + b²)^3/2]i. 
Step-by-step explanation:
So, without mincing words let's dive straight into the solution to the question above. There is need to determine the electric field on-axis of a uniformly-charged disk sitting. 
The electric field in the x-component, dεₓ = 1/4πε₀[H/ x² + b²] cos .
Thus, the total electric field in the x-component, εₓ = 1/4πε₀ [ xdH/ (x^2 + b^2)^3/2. 
Therefore, the electric field = 1/4πε₀ [ xdH/ (x^2 + b^2)^3/2i. 
Where x=0