a) \(M=\left(3x^3+xy-2\right)-\left(2x^3+xy-1\right)\)
\(M=3x^3+xy-2-2x^3-xy+1\)
\(M=\left(3x^3-2x^3\right)+\left(xy-xy\right)+\left(-2+1\right)\)
\(M=x^3-1\)
b) Ta có \(M=-28\)
\(\Leftrightarrow x^3-1=-28\Rightarrow x^3=-27\)
\(\Leftrightarrow x=-3\)