I am in my first year of calculus at the high school level so be patient with me. Why is it that in Lebnit's (spelling?) notation the second derivative is expressed as d²y/dx² ? My instructor did not know and from what we could work out it seems as if it should be expressed as d²y/d²x². We have covered about what you have covered in the first eighth of a college level calculus course so try not to go too far over my head.