To add the two expressions, (-2.9a + 6.8) and (4.4a - 7.3), we should combine like terms. Here, like terms are terms that contain the same variable raised to the same power. We can add the coefficients of 'a' together and the constants together.
Let's start with the 'a' terms:
(-2.9a) + (4.4a).
When we add the coefficients of 'a' together, we get:
-2.9 + 4.4 = 1.5.
So the 'a' terms add up to 1.5a.
Now let's add the constants together:
6.8 + (-7.3).
When performing this addition, we get:
6.8 - 7.3 = -0.5.
So the constant terms add up to -0.5.
Putting both parts together, the result of adding the two expressions is:
1.5a - 0.5.
Therefore, the correct answer is D. 1.5a - 0.5.