\[a \cdot ba^{2} + b^{2}a \cdot 4a =\]
\[= a^{3}b + 4a^{2}b^{2} = a^{2}b(a + 4b);\]
\[a = 2;\ \ b = - \frac{1}{2}:\]
\[2^{2} \cdot \left( - \frac{1}{2} \right) \cdot (2 + 4 \cdot \left( - \frac{1}{2} \right) =\]
\[= - 2 \cdot (2 - 2) = 0.\]