Let's solve the equation step by step.
You started with the equation:
\[ 6x + 3 = 5(x + 3) \]
Expanding the right side gives:
\[ 6x + 3 = 5x + 15 \]
Next, you correctly subtracted \(5x\) from both sides to get all the x terms on the left:
\[ 6x - 5x + 3 = 15 \]
This simplifies to:
\[ x + 3 = 15 \]
Now, to isolate \(x\), subtract 3 from both sides:
\[ x + 3 - 3 = 15 - 3 \]
This simplifies to:
\[ x = 12 \]
Therefore, the solution for \(x\) is:
\[ \boxed{12} \]