5 / (6a) + 2 / (3b)
A common denominator has to have an 'a', a 'b', and a number that contains both 6 and 3; so, let's use 6ab.
The first denominator needs a 'b' and the second denominator needs '2a'; so multiply:
[ 5 / (6a) ] x [ b / b ] + [ 2 / (3b) ] x [ 2a / 2a ]
( 5b ) / ( 6ab) + ( 4a ) / ( 6ab) Add the numerators; place that answer over the common denominator.
= ( 5b + 4a ) / ( 6ab )