Question

18a+-2a+-10a + -10b+12b+6b

Answer 1

just add/subtract the numbers with the same letters

[18-2-10]a + [-10+12+6] b

6a + 8b

Answer 2

we will separate the numbers

18a+-2a+-10a

-10b+12b+6b

RULES: 
+ with + = +
- with - = +
+ with - = -

we will have 18a-2a-10a (we applied the last rule)=
6a

-10b+12b+6b= it's not wrong if we write
12b-10b+6b=8b

Putting all togheter, we will have
6a+8b