Without parenthesis I read this as 3/(4y^{3}) + (2y)/(6y^{2}).

ADDING: find common denominators. Both have a y in the denominator but have different exponents & different coefficients. To get the same coefficients you must multiply the top & bottom by a number which will make the coefficients the same. Simplest way is to say one has a 4 and the other has a 6, multiply each by the opposite number.

The first part: (6/6) x 3/(4y^{3}) = 18/(24y^{3}) The second part (4/4) x (2y)/(6y^{2}) = (8y)/(24y^{2})

The second part can be multiplied by y/y, y/y x (8y)/(24y^{2}) = (8y^{2})/(24y^{3})

the result is 18/(24y^{3}) + (8y^{2})/(24y^{3}) = (18+8y^{2})/(24y^{3}), Now simplify by factoring a 2 from the top and bottom.

(2(9+4y^{2})/(2(12y^{3}) = (9+4y^{2})/(12y^{3})

My laptop is about to die so I will be back later to answer more. I hope this is step by step enough to help you see everything. Remember adding and subtracting require common denominator.