How to save function to calculate in an equation?

How do you save dydx so it's not a function and can be calculated in error?

At the moment, I get the error: "Undefined operator '-' for input arguments of type 'function_handle'." And it seems because my dydx is a function, it cannot be calculated.

Answer 1

[Steven Lord]

You can't add a function handle and a number.

You can add the value you receive from evaluating a function handle and a number.