a) \(M=\left(3x^3+3x^2y-3xy^2+xy\right)-\left(2x^3+3x^2y-3xy^2+xy-1\right)\)
\(M=3x^3+3x^2y-3xy^2+xy-2x^3-3x^2y+3xy^2-xy+1\)
\(M=\left(3x^3-2x^3\right)+\left(3x^2y-3x^2y\right)+\left(3xy^2-3xy^2\right)+\left(xy-xy\right)+1\)\(M=x^3+1\)
b)\(M=9\Leftrightarrow x^3+1=9\)
\(x^3=8\)
\(x^3=2^3\Rightarrow x=2\)
Vậy với x=2 thì M=9