ANSWER
[tex]21x^8[/tex]
EXPLANATION
We want to rewrite the expression in simpler exponent form.
To do this, first, separate the numerical terms from the algebraic terms:
[tex]\begin{gathered} 3*7*x^3*x^5 \\ 21*x^3*x^5 \end{gathered}[/tex]
Now, apply the following law of exponents:
[tex]x^a+x^b=x^{a+b}[/tex]
Therefore, the expression becomes:
[tex]\begin{gathered} 21*x^{3+5} \\ 21*x^8 \\ 21x^8 \end{gathered}[/tex]
That is the answer.
