Adding/Subracting Fractions with Variables

I need some help understanding whats going on here.

Description:

When adding fractions with variables in one or more denominators, the LCD will have each variable to its highest power as a factor.

Can someone please help me through this process, I understand picking out the highest exponent of each variable but after that its unclear. Also how do you know to arrange the expression? For instance How would I know that the gets matched up with the and not on the other end of the expression with the ?

Many Thanks