The way I read your problem, something that goes from 42 to 35 shows a decrease. Therefore the percentage is negative.
To find the answer when rounding to the nearest tenth, you would consider the digit in the hundredth place. In this case, 16.666666 repeating, the digit in the hundredth place is 6. Since 6 is equal to or greater than 5, you would round up to the next whole number in the tenths place. Therefore, the answer rounded to the nearest tenth would be 16.7.