For this question, we recall that a perfect square is of the form:
[tex](a+b)^{2}[/tex]Therefore, we will add and subtract constants in order to arrive at the perfect square form:
[tex]\begin{gathered} 9=x^{2}-8x \\ 9=x^{2}-8x+16-16 \\ 9+16=x^{2}-8x+16 \\ 25=(x-4)^{2} \end{gathered}[/tex]