Answer
$$-t^7+20t^6$$
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:
$$-t^7+12t^6+8t^6-t^3+4t^3-3t^3 \\ -t^7+20t^6$$
