Is what you have written exactly the way the question was written because it looks like to me that you already have answers to part (i) and (ii).
Looking a sample exam question...
(i) Write down the general form of a semi linear first order PDE in u(x,y)
(ii) Write down the ordinary differential equation satisfied by a characteristic curve in the xy plane for your pde in (i)
(iii)Give a careful consideration of the ordinary differential equation satisfied by u(x,y) along such a characteristic curve?
I have not found anything in my notes regarding this...May anyone help?