Answer
$$-1.1a^2+5.3a-7.5$$
Work Step by Step
Recall, like terms must (1) be raised to the same power and (2) consist of the same variable (i.e. x and y are not like terms). It is also important to note that numbers are all like terms with each other when not multiplied by anything. Doing this, we find:
$$8.3a^2-9.4a^2+3.7a+1.6a-8+0.5 \\ -1.1a^2+5.3a-7.5$$
