Question

(a-1)-(a-2)+(a-3)= ?

Answer 1

First of all you need to get rid of parenthesis:
$(a-1)-(a-2)+(a-3)=a-1-a+2+a-3$

Then sum members of expression:
$a-1-a+2+a-3=(a - a + a) + (2 - 1 - 3)=a-2$

So, $(a-1)-(a-2)+(a-3)=a-2$

Answer 2

(a-1)-1(a-2)+(a-3)=a-1-a+2+a-3=a-2
The answer is a-2