1. Problem statement: The exponent on 3 is $x+4$, not $x$. 2. Formula and rule: For any base $a$ and exponents $m,n$ we have $a^{m+n}=a^m a^n$. 3. Apply the rule: Using $a=3$, $m=x$, $n=4$ we get $3^{x+4}=3^x 3^4$. 4. Compute the integer power: $3^4=81$ so $3^{x+4}=81\cdot 3^x$. 5. Final answer: $3^{x+4}=81\cdot 3^x$.