To solve these problems, we need to find a common denominator for the fractions and then perform the arithmetic operations.

### Problem a) \(-\frac{3}{14} + \frac{8}{35}\)

1. **Find the Least Common Denominator (LCD):**
- The denominators are 14 and 35.
- The prime factors of 14 are \(2 \times 7\).
- The prime factors of 35 are \(5 \times 7\).
- The LCD is the least common multiple (LCM) of 14 and 35, which is \(2 \times 5 \times 7 = 70\).

2. **Convert each fraction to have the LCD as the denominator:**
- For \(-\frac{3}{14}\):
\[
-\frac{3}{14} = -\frac{3 \times 5}{14 \times 5} = -\frac{15}{70}
\]
- For \(\frac{8}{35}\):
\[
\frac{8}{35} = \frac{8 \times 2}{35 \times 2} = \frac{16}{70}
\]

3. **Add the fractions:**
\[
-\frac{15}{70} + \frac{16}{70} = \frac{-15 + 16}{70} = \frac{1}{70}
\]

So, the result of \(-\frac{3}{14} + \frac{8}{35}\) is \(\frac{1}{70}\).

### Problem b) \(\frac{3}{5} - (-\frac{8}{25})\)

1. **Simplify the expression:**
- Subtracting a negative is the same as adding the positive:
\[
\frac{3}{5} - (-\frac{8}{25}) = \frac{3}{5} + \frac{8}{25}
\]

2. **Find the Least Common Denominator (LCD):**
- The denominators are 5 and 25.
- The LCD is 25 (since 25 is a multiple of 5).

3. **Convert each fraction to have the LCD as the denominator:**
- For \(\frac{3}{5}\):
\[
\frac{3}{5} = \frac{3 \times 5}{5 \times 5} = \frac{15}{25}
\]
- For \(\frac{8}{25}\):
\[
\frac{8}{25} = \frac{8}{25}
\]

4. **Add the fractions:**
\[
\frac{15}{25} + \frac{8}{25} = \frac{15 + 8}{25} = \frac{23}{25}
\]

So, the result of \(\frac{3}{5} - (-\frac{8}{25})\) is \(\frac{23}{25}\).