How is the missing number determined to make a perfect square?

Explanation
a perfect square has the form
[tex]a^2+2ab+b^2[/tex]so, the thir t term would be
[tex]\begin{gathered} b=\frac{2ab}{2\sqrt{a^2}} \\ \sqrt{thirterm}=\frac{second\text{ term}}{2*\sqrt{first\text{ term}}} \end{gathered}[/tex]so
Step 1
[tex]v^2+6v[/tex][tex]v^2+6v\Rightarrow a^2+2ab[/tex]hence
[tex]\begin{gathered} \sqrt{b}=\frac{second\text{term}}{2\sqrt{first\text{ term}}} \\ \sqrt{b}=\frac{6v}{2\sqrt{v^2}} \\ \sqrt{b}=\frac{6v}{2v^} \\ \sqrt{b}=3 \\ \sqrt{b}=3 \\ b=3^2 \\ b=9 \end{gathered}[/tex]therefore, the answer is
9
I hope this helps you