Hi there!
Here are all the steps into simplifying this :
Since "-2a" and "3a" share the same variable, which is "a", you can add both coefficients to be left with just one variable instead of two :
-2 + 3 = 1
Since the coefficient of "a" would be 1, you can just write "a", which is the same thing.
So the simplified expression would be : a + 6
There you go! I really hope this helped, if there's anything just let me know! :)