Thank you for the reply, but there is a square root at the denominator (y^2+ (x-z)^2)^1/2.
I know the answer of this derivation, with respect to limits 0 to L,
Integral of
dz/ Square root of (y^2+(x-z)^2) =
ln (x+ Square root of (x^2+y^2)/ x-L + square root of...