MATLAB: How to count derivative and then put a value inside it

derivativedifferential equationsfunctionMATLAB

So what I'm doing now is
syms x;
fx = tikslo(x);
dfx = diff(fx, x);
when function "tikslo" is
global tiksloIter;
tiksloIter = tiksloIter + 1;
y = (((x^2 - 7)^2 )/3) - 1;
The derivative I get is dfx = (4*x*(x^2 – 7))/3, but how do I put x value into it?
How to do dfx(5) for example so that I would get answer what that derivative with 5 is equal to?
Right now if I try to do that I get "Index exceeds matrix dimensions." and "Error in sym/subsref (line 881)"

Best Answer

 double( subs(dfx,5) )
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