Okay, here we have this:
Considering the provided function, we are going to convert it to vertex form by completing the square to identify what number should Megan use to replace the a in the equation, so we obtain the following:
Then we will first write the function in the form: x^2+2ax+a^2. We have:
2a=-10
a=-10/2
a=-5
Then:
![\begin{gathered} x^2-10x+3+(-5)^2-(-5)^2 \\ =(x-5)^2+3-(-5)^2 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/4c75d8wncppa7ljfyx8hqxjay9pcvh1chl.png)
Finally we obtain that Megan should use "-5" to replace the a in this equation.