To solve the equations in the problem, let's break it down:

### a) \( \frac{10.6}{x} + \frac{7.19}{x} = 3 \)

1. Combine the fractions:
\[
\frac{10.6 + 7.19}{x} = 3
\]
\[
\frac{17.79}{x} = 3
\]

2. Cross-multiply:
\[
17.79 = 3x
\]

3. Solve for \( x \):
\[
x = \frac{17.79}{3} \approx 5.93
\]

### b) \( \frac{7}{x} - \frac{3.9}{x} = 5 \)

1. Combine the fractions:
\[
\frac{7 - 3.9}{x} = 5
\]
\[
\frac{3.1}{x} = 5
\]

2. Cross-multiply:
\[
3.1 = 5x
\]

3. Solve for \( x \):
\[
x = \frac{3.1}{5} = 0.62
\]

### Summary of Solutions:
- For part a, \( x \approx 5.93 \)
- For part b, \( x = 0.62 \)