${6}^{3}={\left( \frac{ 1 }{ 6 } \right)}^{x+3}$
Express with a positive exponent using ${\left( \frac{ 1 }{ a } \right)}^{-n}={a}^{n}$${6}^{3}={6}^{-x-3}$
Since the bases are the same, set the exponents equal$3=-x-3$
Move the variable to the left-hand side and change its sign$3+x=-3$
Move the constant to the right-hand side and change its sign$x=-3-3$
Calculate the difference$x=-6$