a) `A vdots B <=> {(x^(n-1) vdots x^2),(y^4 vdots y^n):} <=> {(n-1>=2),(4>=n):} <=> {(n>=3),(n<=4):} <=> 3<=n<=4`
`=> n in {3;4}`
b) `M vdots N <=> {(x^(n+1) vdots x^3),(y^2 vdots y^(n-1)):}<=>{(n+1>=3),(2>=n-1):}<=>{(n>=2),(n<=3):}<=>2<=n<=3`
`=> n in {2;3}`