To add the fractions \(-\frac{12}{7}\) and \(\frac{3}{14}\), we need a common denominator.
The least common denominator (LCD) for \(7\) and \(14\) is \(14\).
Now, we convert \(-\frac{12}{7}\) to have the denominator \(14\): \[ -\frac{12}{7} = -\frac{12 \times 2}{7 \times 2} = -\frac{24}{14} \]
Now we can add the two fractions: \[ -\frac{24}{14} + \frac{3}{14} = \frac{-24 + 3}{14} = \frac{-21}{14} \]
Next, we will simplify \(\frac{-21}{14}\). The greatest common divisor (GCD) of \(21\) and \(14\) is \(7\): \[ \frac{-21 \div 7}{14 \div 7} = \frac{-3}{2} \]
Thus, the answer is: \[ \boxed{-\frac{3}{2}} \]