I am stuck on a problem and cant figure out how to go about it.

I have a unit speed curve in a helicoid parametrized (z,x)->(z cos x, z sin x, x)

I have found the first fundamental form of the unit speed curve in the helicoid. the coefficients being 1,0 and (1+z^2).

I now need to show that x' = c/((1+z)^2) for some constant c but I dont know how to do this, any ideas or help to put me in the right direction would be much appreciated.