With regard to functions, we must know the meaning of independent variables. For f(x), it can be said that x is an alternative representation of a given unknown quantity.

So f(-a) means: "When x=-a, …" that is, changing x into -a in the function. (that is, the notes you made under the topic refer to)

Another thing to note about functions is the domain problem. Remember! When you see an unknown number, you should consciously examine its range of values.

As for the fifth question, which is related to practical problems, we must consider the actual situation. If it is required to be bent into a "frame", each side must be greater than 0, otherwise it does not exist. So are other similar topics.