To solve the expression for \( P \), we need to perform the arithmetic operations step by step. Let's break it down:

First, calculate the values in the numerator:

\[ 2.5 \times 4.3 = 10.75 \]

\[ 1.7 \times 7.8 = 13.26 \]

\[ 3.5 \times 2.5 = 8.75 \]

Now, substitute these values back into the numerator:

\[ 10.75 - 13.26 + 8.75 \]

Perform the subtraction and addition:

\[ 10.75 - 13.26 = -2.51 \]

\[ -2.51 + 8.75 = 6.24 \]

So, the numerator is \( 6.24 \).

Next, calculate the values in the denominator:

\[ 10.4 + 11.5 + 12.6 - 11.8 - 10.7 - 9.6 \]

Perform the addition and subtraction:

\[ 10.4 + 11.5 + 12.6 = 34.5 \]

\[ 11.8 + 10.7 + 9.6 = 32.1 \]

\[ 34.5 - 32.1 = 2.4 \]

So, the denominator is \( 2.4 \).

Finally, divide the numerator by the denominator to find \( P \):

\[ P = \frac{6.24}{2.4} = 2.6 \]

Therefore, \( P = 2.6 \).