ta có \(a+b=3\Rightarrow a=-3b\)

thay \(a=-3b\) vào a+b=3 ta được

\(-3b+b=3\Rightarrow-2b=3\Rightarrow b=\dfrac{-2}{3}\)

\(\Leftrightarrow a=3+\dfrac{2}{3}=\dfrac{11}{3}\)

Ta có

\(a^3+b^3=\left(a+b\right)\left(a^2-ab+b^2\right)=3.\left[5-\dfrac{11}{3}.\left(-\dfrac{2}{3}\right)\right]=3.\left(5-\dfrac{22}{3}\right)=3.\dfrac{-7}{3}=-7\)