The "Mixed Fraction" may be the worst construct of all time. Make it an "Improper Fraction" and you'll be almost done. Don't let the word "Improper" fool you. Excepet for comparing magnitude, it is the best form for just about everything.
(-2/3) + (3 and 1/3) = (-2/3) + (9/3 + 1/3) = (-2/3) + (10/3) = (-2 + 10)/3 = 8/3
I think we're done. Sadly, someone may want that nasty mixed fraction. Fine 2 and 2/3.