312 rounded to the nearest tens would be 310. There are no tenths in the given number.
What does it have to do with Pythagoras? But, for the record, in a right-angled triangle Pythagoras' Theorem states that the square on the hypotenuse=sum of the squares on the other two sides, usually shown as a^2=b^2+c^2 where a is the length of the hypotenuse. The smallest unit length right-angled triangle is 3, 4, 5, where 5 is the unit length of the hypotenuse.