Question 331748
Subtract {{{(2x+n)^2}}} from boths sides:

{{{4x^2+20x+n^2-(2x+n)^2}}}

Expand out and simplify and it whittles down to

{{{20x-4nx=0}}}

Factor out x:

{{{x(20-4n)=0}}}

{{{20-4n=0}}} when n=5.

It is true for all x when n=5.