50g

Hello there, what is happening with your equations is that the F(x) in N
(x) is different from the F(x) you define, for example when you define f
(x)=x^3-7 the calculator creates a program that looks like this:
<< ¨ x 'x^3-7' >>
and stores it in a global variable called 'F'; this means that in run-
time the calculator create a temporary local variable called 'x' this 
variable is different that a global variable called 'x' that you may 
have in your current directory. The same is true for the F(x) you 
define and F(x) in N(x). A better approach is to have a program N like 
this:
<< ¨ x0
    << 'x' DUP F DUP 'x' Ý / - 'x' x0 = SUBST ¨NUM >>
>>
I hope this helps.