How do I do this question on trigo...

sin(-720-A)
= sin (-360-360-A)
=-sinA
I got this question right, but I don't know if that's the right way to do it..

(Assuming everything is in degrees)

Technically that is a correct way to show it.

This is because:
sin (k*360 + x) = sin (x)
and also:
sin (k*-360 + x) = sin(x)
where k is an integer

I guess if you want it a little better you could add a couple of steps between lines 2 and 3:
= sin (-360-360-A) <---Original line 2
= sin (-360 - A)
= sin (-A)
= -sinA <---Original line 3