1st: you need to make a common denominator for 2/3 and 11/12 that would be 12, so you would keep 11/12 how it is but multiply the numerator and the denominator by 4 to get it to 12.
For a function to have a derivative in a point has to be continuous at the point, that is, it has to be defined at that point, other wise the derivative would be meaningless.
