So, I am making a program for Linear Approximations, I need to take the Derivative of the function. So I have 3 inputs, a, fx, and dx, where fx is a functions. However, when you try to input a function, it says variable not defined, which is quite annoying, and causing my program to be useless.

Here is my code:

```
Define LibPub pprox(a,fx,dx)=
Prgm
:Local deriv
:Local approxi
:Local function
:deriv:=(fx,x)
:approxi:=dx*deriv(a)
:Return approxi
:EndPrgm
```

That isnt the completed program, it will be easy once I can input the function. Thanks guys!