Question 407342
{{{81x^2-54xy+9y^2}}}First factor out the GCF 9. 
{{{9(9x^2-6xy+y^2)}}}Then factor this polynomial, which is considerably easier.
{{{9(3x-y)(3x-y)}}} Look for 2 numbers that mult to 9x^2: 3x and 3x (it could be 9x and x, but it doesn't work out), they go in the first spot in the ().  Then look for 2 numbers that mult to get y^2: y and y, they go in the last spot in the (). Then both factors get negative signs because if the last sign in the polynomial is + then the signs are the same and then you use whatever the first sign was, -.  Also check by FOILing it back to make sure you factored it right.
I'm assuming you know something about factoring. If not, then this is not the one to learn on.