Let's solve each part step by step:

### a) \( 1,5 - 3^2 + 7,5 : 5 \)
1. Calculate \( 3^2 \):
\[ 3^2 = 9 \]
2. Perform the subtraction:
\[ 1,5 - 9 = -7,5 \]
3. Perform the division:
\[ 7,5 : 5 = 1,5 \]
4. Add the results:
\[ -7,5 + 1,5 = -6 \]

### b) \( 0,3 + 2,5 \cdot 4 + (7,3 - 1) : 9 \)
1. Calculate \( 2,5 \cdot 4 \):
\[ 2,5 \cdot 4 = 10 \]
2. Calculate \( 7,3 - 1 \):
\[ 7,3 - 1 = 6,3 \]
3. Perform the division:
\[ 6,3 : 9 = 0,7 \]
4. Add the results:
\[ 0,3 + 10 + 0,7 = 11 \]

### d) \( 0,75 + \frac{9}{45} + 0,8 + \frac{5}{20} \)
1. Simplify the fractions:
\[ \frac{9}{45} = 0,2 \]
\[ \frac{5}{20} = 0,25 \]
2. Add the results:
\[ 0,75 + 0,2 + 0,8 + 0,25 = 2 \]

### e) \( \frac{15}{12} - \left( \frac{5}{13} \right) - \frac{3}{12} \cdot \frac{18}{13} \)
1. Simplify the fractions:
\[ \frac{15}{12} = 1,25 \]
\[ \frac{3}{12} = 0,25 \]
2. Multiply the fractions:
\[ 0,25 \cdot \frac{18}{13} = \frac{4,5}{13} \approx 0,346 \]
3. Subtract the results:
\[ 1,25 - \frac{5}{13} - 0,346 \approx 1,25 - 0,385 - 0,346 = 0,519 \]

### g) \( \frac{5}{6} : \frac{1}{3} - \frac{8}{21} : \frac{4}{7} \)
1. Perform the divisions:
\[ \frac{5}{6} : \frac{1}{3} = \frac{5}{6} \cdot 3 = 2,5 \]
\[ \frac{8}{21} : \frac{4}{7} = \frac{8}{21} \cdot \frac{7}{4} = \frac{8 \cdot 7}{21 \cdot 4} = \frac{56}{84} = \frac{2}{3} \]
2. Subtract the results:
\[ 2,5 - \frac{2}{3} = 2,5 - 0,666 = 1,834 \]

### h) \( \left( \frac{3}{5} \right)^2 - 2 \cdot \frac{3}{4} : \left( 0,2 + \frac{18}{27} \right) \)
1. Calculate \( \left( \frac{3}{5} \right)^2 \):
\[ \left( \frac{3}{5} \right)^2 = \frac{9}{25} = 0,36 \]
2. Multiply the fractions:
\[ 2 \cdot \frac{3}{4} = \frac{6}{4} = 1,5 \]
3. Simplify the fraction:
\[ \frac{18}{27} = \frac{2}{3} \approx 0,666 \]
4. Add the results:
\[ 0,2 + 0,666 = 0,866 \]
5. Perform the division:
\[ 1,5 : 0,866 \approx 1,732 \]
6. Subtract the results:
\[ 0,36 - 1,732 = -1,372 \]

So, the final results are:
a) -6
b) 11
d) 2
e) 0,519
g) 1,834
h) -1,372