To factor 81p² + 90p + 25, find 81 and 25's factors that would be added up to 90.
We could use 9 and 5.
(9p + 5)(9p + 5)
To check if we factored it right, we can solve (9p+5)(9p+5).
81p² + 45p + 45p + 25
81p² + 90p + 25
So your answer is
(9p + 5)(9p + 5) or (9p + 5)².
Hope this helped :)