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‘X’ is not the variable, it is merely one of two parameters.
The slope is the variable sought.
More coffee help?
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The slope is the variable sought.
The slope is itself a function of x, a single variable.
You strike me as someone who gets irate when he is caught in a silly mistake because he doesn't like to be found wrong. On the other hand, I like to imagine that when the author of my university calculus textbooks titled them Single-Variable Calculus and Calculus of Several Variables, he understood the subject matter well enough to know the former actually exists. Then again, it was only a top engineering school, so what do they know about math.