We seem to have two sets of brackets and one on its own. Let's forget about the lone curly bracket because we don't know where its partner is. The innermost bracket is (2+1) making 3. Remember this result. Now we move to the next one: before we get to the square brackets we have 10-3, but we have to assume that multiplication takes priority over subtraction, therefore we need to multiply the result of the innermost brackets by 3 before subtracting from 10. So it's 10-3*3=10-9=1. It could be (10-3)*3*3, but the spacing of the characters isn't enough to tell us that. In other words "10-3 x (2+1)" isn't the same as "(10-3)x (2+1)" (that would have given us 21). So we are justified in doing the multiplication before the subtraction. OK, now we have the contents of the square brackets equal to 1. Remember this result. Outside the square brackets we have 3+6+1=10 and it doesn't matter which way we add them to the result of the square brackets, it's going to be the same: 10+1=11. You can now see why you got the answer 31, because you interpreted the contents of the square brackets as 21 instead of 1. The spacing of the characters tripped you up!