How can I make these two circles the same size? please help me

It is recommended that you post a minimal working example (MWE) that starts with documentclass and ends with end{document}.

Anyway, I came up with

documentclass{standalone}
usepackage{tikz}

begin{document}
begin{tikzpicture}
node[circle, draw=black, solid] (Q9) at (10.5,-1.5){phantom{Q10}};
node at (Q9) {Q9};
node[circle, draw=black, solid] (Q10) at (9.0, -1.5){Q10};
end{tikzpicture}
end{document}

which produces the next output:

Notice how I put {phantom{Q10}} in the first node to get the spacing of Q10, and in the line below, I place the text Q9 at the coordinate (Q9).