Start at $0$ then move $a$ units to the right, then move $a$ units to the left. The ending position is the sum of $a+(-a)$.
Work Step by Step
Starting at $0,$ go to the right by $a$ units. To add $-a,$ start at $a$ then go to the left by $a$ units. The ending point (which is at $0$) is the sum of $a+(-a).$