Using a predefine function into a function

Hi everyone,

I need to resolve a 1-variable equation into a function. For that, I want to use the function 'Trouver', which might be 'Find' in English, to resolve my equation.

The problem is that the function 'Trouver' is 'unknown' when I use it into my own function.

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'Soit' means 'Suppose that'.

Does anyone as an idea of what's wrong in my function ?

TIA