If you mean a^4 - 26a^2 + 169
then it looks like you have a perfect square.
The fact that the first and last terms were perfect squares gave me that clue.
Give it a try, it is quite simple
a4-26a2+169
2 answers
sqrt a^4 = a^2.
Sqrt 169 = 13.
solution: (a^2 - 13)^2.
check:
(a^2 - 13)^2 = a^4 - 26a^2 + 169.
Sqrt 169 = 13.
solution: (a^2 - 13)^2.
check:
(a^2 - 13)^2 = a^4 - 26a^2 + 169.