To solve this question, we must first rewrite one or more of the expressions so they have the same base

We can do this by replacing $8$ with $2^3$ to get: $(2^3)^{2x+3}=2^{3x+6}$

Now apply the power of a power law to the left side to get: $2^{6x+9}=2^{3x+6}$

Now that we have the same bases, we know the exponents must be equal: $6x+9=3x+6$

Subtract $3x$ from both sides of the equation: $3x+9=6$

Subtract $9$ from both sides of the equation: $3x=-3$

Divide both sides by $3$ to get: $x = -1$

Answer: B