Jean-March - I suspect that the inputs F and DF are function handles to the function and its first derivative. For example,
f = @(x)x^2 + 3*x;
df = @(x)2*x + 3;
where df is the first derivative of f. As these are function handles, we can evaluate them for any value of x. For example, if x is 42, then we can write
f(42)
df(42)
to evaluate f and df are 42. For Newton's Method, you would use these two functions as
xnl = xn - f(xn)/df(xn);
where xn is the value calculated on the previous iteration (or x0 if first the first iteration).
Please be careful when naming your variables - you attempt to use an fx but it is never assigned. Should this be Fx instead? Also, is delta_x an appropriately named variable?
Try implementing the above into your code and see what happens!
