Order of operations is useful here: PEMDAS (Parenthesis, Exponents, Multiplication and Division, Addition and Subtraction.)
((-3) - 3) - 4
So that nested (-3) all by itself is equal to -3:
((-3) - 3) - 4
(-3 - 3) - 4
Then we evaluate the expression inside the parenthesis, (-3 - 3), which equals -6:
(-3 - 3) - 4
(-6) - 4
Then we can turn the (-6) by itself into just plain -6:
(-6) - 4
-6 - 4
And evaluate -6 - 4, which equals -10:
-6 - 4
-10