The square brackets are just to make it easier to read, so that you can match open and closed brackets (parentheses). (75/15)=5 and (-10+15)=5, and 5-5=0. Any product with zero as one of the multipliers is zero. So we don’t need to work out the contents of the first pair of square brackets. In this problem [ ... ][ ... ] means: find the product of the contents of the first pair of brackets and the contents of the second pair (which came to zero). So we’re left with 0-(19/3)=-19/3.