The associative law for addition says that the order you carry out in adding quantities doesn't matter: the result is always the same. So we can change the order in the given calculation to make it easier.
[(540)+(-540)]+[(1560)+(-60)]=(540-540)+(1560-60)=0+1500=1500.
Now, using the given order:
(-540+1560)+(540-60)=1020+480=1500.
And we can try other orders, but the result will always be 1500.