To simplify this expression you have to apply the power of a power rule, which implies that you have to multiply the power indices when an exponential number is raised to another power:
[tex]\begin{gathered} (-4x^2ya^3)^2=(-4)^2\cdot(x^2)^2\cdot(y)^2\cdot(a^3)^2 \\ 16\cdot x^{2\cdot2}\cdot y^{1\cdot2}\cdot a^{3\cdot2} \\ 16x^4y^2a^6 \end{gathered}[/tex]
