Let's solve the given expressions step by step.

### a)
The expression is:
\[ \left( \frac{9}{15} \times \frac{7}{3} \times \frac{25}{9} \right) \times 0.81 \]

First, simplify each fraction:
\[ \frac{9}{15} = \frac{3}{5} \]
\[ \frac{7}{3} \text{ (already in simplest form)} \]
\[ \frac{25}{9} \text{ (already in simplest form)} \]

Now, multiply the fractions:
\[ \frac{3}{5} \times \frac{7}{3} \times \frac{25}{9} \]

First, multiply \(\frac{3}{5}\) and \(\frac{7}{3}\):
\[ \frac{3}{5} \times \frac{7}{3} = \frac{3 \times 7}{5 \times 3} = \frac{21}{15} = \frac{7}{5} \]

Next, multiply \(\frac{7}{5}\) and \(\frac{25}{9}\):
\[ \frac{7}{5} \times \frac{25}{9} = \frac{7 \times 25}{5 \times 9} = \frac{175}{45} = \frac{35}{9} \]

Now, multiply the result by 0.81:
\[ \frac{35}{9} \times 0.81 \]

Convert 0.81 to a fraction:
\[ 0.81 = \frac{81}{100} \]

Now multiply:
\[ \frac{35}{9} \times \frac{81}{100} = \frac{35 \times 81}{9 \times 100} = \frac{2835}{900} \]

Simplify the fraction:
\[ \frac{2835}{900} = \frac{315}{100} = 3.15 \]

So, the value of expression a) is:
\[ 3.15 \]

### b)
The expression is:
\[ \frac{6}{13} \times \frac{1}{4} \times \frac{14}{26} \times \frac{1}{4} \]

First, simplify each fraction:
\[ \frac{6}{13} \text{ (already in simplest form)} \]
\[ \frac{1}{4} \text{ (already in simplest form)} \]
\[ \frac{14}{26} = \frac{7}{13} \]
\[ \frac{1}{4} \text{ (already in simplest form)} \]

Now, multiply the fractions:
\[ \frac{6}{13} \times \frac{1}{4} \times \frac{7}{13} \times \frac{1}{4} \]

First, multiply \(\frac{6}{13}\) and \(\frac{1}{4}\):
\[ \frac{6}{13} \times \frac{1}{4} = \frac{6}{52} = \frac{3}{26} \]

Next, multiply \(\frac{3}{26}\) and \(\frac{7}{13}\):
\[ \frac{3}{26} \times \frac{7}{13} = \frac{21}{338} \]

Finally, multiply \(\frac{21}{338}\) and \(\frac{1}{4}\):
\[ \frac{21}{338} \times \frac{1}{4} = \frac{21}{1352} \]

So, the value of expression b) is:
\[ \frac{21}{1352} \]