Expand the binomial: y = (x-3)^4
(x-3)^2 = x^2-6x+9
now multiply that by itself in steps
x^2(x^2-6x+9)=x^4-6x^3+9x^2
-6x(x^2-6x+9)=-6x^3+36x^2-54x
9(x^2-6x+9)=9x^2-54x+81
now add those right hand sides together to get the answer.
now multiply that by itself in steps
x^2(x^2-6x+9)=x^4-6x^3+9x^2
-6x(x^2-6x+9)=-6x^3+36x^2-54x
9(x^2-6x+9)=9x^2-54x+81
now add those right hand sides together to get the answer.