Question 622278
what two numbers multiply together to give you +81 yet add together to give you -18? the answer is -9. so this equation becomes,
{{{y=(x^2-9x)+(-9x+81)}}}

well now you can factor out an x from the first bracket and -9 from the second bracket and you will get,
{{{y=x(x-9)-9(x-9)}}}
now (x-9) is a common factor. so the equation becomes,
{{{y=(x-9)(x-9)}}} 
this is a perfect square, so it is now written as, 
{{{y=(x-9)^2}}}

hope this helps!