A ponit charge q is kept at the centre of the cylinder of length L and radius a> calculate the flux throuugh the curved surface of the cylinder

Consider the x axis to be along the axis and point charge at the origin.

Now take an area element at height x above the charge dA= 2πrdx

where r= (R^2+x^2)^1/2

and normal component of electric field will be Ecosø where E=(1/4π€)q/r^2

and cosø=R/r

now integrate this (scalar) .. from x=-h to x=+h  and you will get the answer.