In order to multiply terms with the same base, we can use the following property:
[tex]a^b\cdot a^c=a^{b+c}[/tex]So we have that:
[tex]\begin{gathered} 5^3\cdot x^2\cdot x^{-3}\cdot5^3\cdot x^4 \\ =5^3\cdot5^3\cdot x^2\cdot x^{-3}^{}\cdot x^4 \\ =5^{3+3}\cdot x^{2-3+4} \\ =5^6\cdot x^3 \end{gathered}[/tex]