Explanation:
that only means we need to put the given values into every place, where the variable is used in the expression.
this is straight forward.
x² for x = -9 means
(-9)² = 81
you remember, a² = a×a.
and when a is negative, this is then the multiplication of a negative number with a negative number, and that result is positive.
so, a square operation always delivers a positive result for real numbers as input, no matter if the input was already positive or negative.