What does this mean, [f(x)]^2

Ok so I'm doing simple f(x) evaluating functions and I came across this, what does this mean when you substitute into this Let f (x) = 7x − 13.

[f(x)]^2=?

I know for example,
f(7x)= 49x-13
or
f(x^2)= 7x^2-13
but [f(x)]^2 gets me. Help please.

It's probably just a grouping symbol, so in this case:

[f(x)]^2 = [7x - 13]^2 = 49x^2 - 182x + 169

Just substitute the expression, keeping it in brackets, and simplify from there.

Just square the function of x

Example: f(x) = x +1; [f(x)]^2 = x^2 + 2x + 1