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:

Finally we obtain that Megan should use "-5" to replace the a in this equation.