15. \( -\frac{3}{4}-\frac{1}{8} \)

To solve \( -\frac{3}{4}-\frac{1}{8} \), first, we need a common denominator. The least common multiple of 4 and 8 is 8. Convert \( -\frac{3}{4} \) to have a denominator of 8. This gives us \( -\frac{6}{8} \). Now, you have: \[ -\frac{6}{8} - \frac{1}{8} = -\frac{6 + 1}{8} = -\frac{7}{8} \] So the final answer is \( -\frac{7}{8} \). To double-check: converting back to a decimal, both fractions yield -0.875, matching our result!