a) \( \frac{12}{25}+\left(-\frac{15}{50}\right)=\square+\frac{12}{25} \)

To solve the equation, first simplify \(-\frac{15}{50}\) to \(-\frac{3}{10}\). This can be expressed as \(-\frac{3}{10} = -\frac{6}{20}\) or \(-\frac{9}{30}\) to have a common denominator. Next, convert \(\frac{12}{25}\) so that both fractions can be added together. We can convert \(\frac{12}{25}\) to a fraction with a denominator of 50: \[ \frac{12}{25} = \frac{24}{50} \] Now substitute this into the equation: \[ \frac{24}{50} + \left(-\frac{15}{50}\right) = \square + \frac{24}{50} \] Calculating the left-hand side gives: \[ \frac{24}{50} - \frac{15}{50} = \frac{9}{50} \] So we have: \[ \frac{9}{50} = \square + \frac{24}{50} \] Next, isolate \(\square\): \[ \square = \frac{9}{50} - \frac{24}{50} = -\frac{15}{50} = -\frac{3}{10} \] Thus, the final answer is: \[ \square = -\frac{3}{10} \]