To solve the equation \( \frac{5}{-12} = \frac{3}{x} \), we can cross-multiply:
\[ 5x = -12 \cdot 3 \]
Calculating the right side:
\[ 5x = -36 \]
Next, divide both sides by 5:
\[ x = \frac{-36}{5} \]
So the solution is:
\[ x = -\frac{36}{5} \] or -7.2 in decimal form.