Computing the error bound of floating-point expression
IEEE 754 gives you the maximum relative error of each operation. You have three differences with a maximum relative error each. Multiplication and division adds the maximum relative error of each operand, plus the maximum relative error of the multiplication and division. 

Then you have a final addition. If the right hand side is very close to the negative of the left hand side of the "+" then the relative error is unlimited. You'd need to analyse how close to -a.y the right hand side can be. 

For absolute error, you convert the relative error of the right hand side into an absolute error.