81x + kx + 25, what values of k makes x a perfect square trinomial?

I think this must be :

81x^2 + kx + 25    if so....we can take the square roots of 81 and 25   =  9 and 5  ...multiply them together = 45  ..... and double this  = 90   and this is "k'  .....

Note

81x^2 + 90x + 25 =

(9x + 5) (9x + 5) =

81x^2 + 45x + 45x + 25  =

81x^2 + 90x + 25