The expression to solve is:

$-\frac{3}{7} + \left(-\frac{3}{4}\right)$

First, let's find a common denominator for $\frac{3}{7}$ and $\frac{3}{4}$. The denominators are 7 and 4, so the least common denominator (LCD) is 28.

Next, convert each fraction:

$-\frac{3}{7} = -\frac{3 \times 4}{7 \times 4} = -\frac{12}{28}$

$-\frac{3}{4} = -\frac{3 \times 7}{4 \times 7} = -\frac{21}{28}$

Now, add the fractions:

$-\frac{12}{28} + \left(-\frac{21}{28}\right) = -\frac{12 + 21}{28} = -\frac{33}{28}$

Thus, the final result is:

$-\frac{33}{28}$

Would you like further details or have any questions?

Here are some related questions:

1. How do you find the least common denominator for two fractions?
2. What are some methods to simplify negative fractions?
3. How do you add fractions with unlike denominators?
4. What are improper fractions, and how can they be converted to mixed numbers?
5. Can you explain the process of adding and subtracting fractions with the same sign?

Tip: When adding or subtracting fractions, always look for a common denominator to simplify the process.