If you were asked to evaluate $x^2$ for $x = -1$

Question

Would you bracket the $x$? I ask this because $-1^2$ is equal to $-1$, but $(-1)^2$ is equal to $1$. Which is valid?

Answer 1

Yes you need to bracket the $x$, so the result is $(-1)^2=1$. When you substitute a variable for its value in an equation you always need brackets around it. However sometimes those brackets are unnecessary.

Answer 2

If $x=-1$, then it $x$ has the value of $-1$, so squaring this value will give you $(-1)^2$.

What you did in $x^2\to-1^2$ instead is not assuming the value of a variable, it's replacing a character representing the variable with a string representing a number and then reinterpreting the expression. Of course, this expression doesn't have to have the same meaning.