To solve the subtraction \((8.5 - 1.8i) - (-1.3 + 2.7i)\), we first distribute the negative sign:
\[ (8.5 - 1.8i) + (1.3 - 2.7i) \]
Now, we can combine the real parts and the imaginary parts separately:
Real part: \[ 8.5 + 1.3 = 9.8 \]
Imaginary part: \[ -1.8 - 2.7 = -4.5 \]
Putting it all together, we get:
\[ 9.8 - 4.5i \]
Thus, the final answer in the form \(a + bi\) is:
\[ \boxed{9.8 - 4.5i} \]