Errors do not sum like regular numbers. You want to take the square root of the sum of the squares of the errors.
sqrt(6.4^2 + 6.9^2) ≈ 9.4
You can have a look here: http://ipl.physics.harvard.edu/wp-uploads/2013/03/PS3_Error_...
sqrt(6.4^2 + 6.9^2) ≈ 9.4
You can have a look here: http://ipl.physics.harvard.edu/wp-uploads/2013/03/PS3_Error_...