<PRE><B>Question 842989</B>
{{{9n^2-30n+c=(3n-k)(3n-k)}}} perfect square
{{{9n^2-3kn-3kn+k^2}}}
{{{9n^2-6kn+k^2}}}


Compare the terms and variables in the corresponding positions, especially the terms of n.
{{{30n=6kn}}}
Using very little or no formal analysis, you expect 30=6k meaning that k=5.


CHECK:
{{{(3n-5)^2=9n^2-15n-15n+25=highlight_green(9n^2-30n+25)}}} Good.
Also, {{{highlight(c=25)}}}</PRE>