How do you factor this perfect square trinomial

81x^2-90x+25

A² + 2AB + B² = (A + B)²

so

81x² - 90x + 25 =

(9x)² + 2*9x*(-5) + (-5)² =

(9x + (-5))² =

(9x -5)²

completing the square or use [-b (+-)underroot(b^2 - 4ac)]/2a
in this case determinant is zero
so your ans is 90/2.(81)

(9x - 5) * (9x - 5)