\(\dfrac{4}{3}=\displaystyle\int_{-a}^{a} [f(x) - g(x)]\, dx \\ \Leftrightarrow \displaystyle\int_{-a}^{a} [(x^2)-(ax)]\, dx =\int_{-a}^{a} x^2 \,dx - \int_{-a}^{a} ax \, dx \\ \Leftrightarrow \left [ \dfrac{2a^3}{3} \right ]_{-a}^a - 0 \Leftrightarrow \left [ \dfrac{2a^3}{3} \right ]_{-a}^a \\ \rightarrow \dfrac{2a^3}{3} = \dfrac{4}{3} \Leftrightarrow a^3=2 \Rightarrow a=\displaystyle\sqrt[3]{2}\)
Probe: \(\displaystyle\int_{-\displaystyle\sqrt[3]{2}}^{\displaystyle\sqrt[3]{2}} [(x^2)-(\displaystyle\displaystyle\sqrt[3]{2}x) ]\, dx = \int_{-\displaystyle\sqrt[3]{2}}^{\displaystyle\sqrt[3]{2}} x^2\, dx=\dfrac{4}{3}\)