To solve for \( x \) in the equation:
\[ -20 = -5 + 3x \]
First, add 5 to both sides:
\[ -20 + 5 = 3x \]
This simplifies to:
\[ -15 = 3x \]
Next, divide both sides by 3:
\[ x = \frac{-15}{3} \]
This simplifies to:
\[ x = -5 \]
Thus, the solution is:
\[ \boxed{-5} \]