Answer :

use the perfect square trinomial method. since 16x² is a perfect square and so is 81y² your answer would be (4x-9y)² when u multiply that out it will give u 16x²-72xy+81y²
[tex]16x^2-72xy+81y^2=(4x)^2-2\cdot4x\cdot9y+(9y)^2=(4x-9y)^2\\\\=(4x-9y)(4x-9y)\\\\\\\\(a-b)^2=a^2-2ab+b^2[/tex]