Theory:

A trinomial algebraic expression is an algebraic expression that has three terms.

The prefix 'tri' in 'trinomial' stands for \(3\) terms. 
To add two trinomials, you must:
 
1) remove the brackets (without changing the signs, because the “\(+\)” sign is in front of the brackets);
2) add the like terms.
Example:
Add trinomials: 
(5x3+3y5y2)+(8x3+5y22y)
 
1) Remove the brackets:
5x3+3y5y2+8x3+5y22y==5x3+3y5y2+8x3+5y22y.
 
2) Find the like terms and add:
5x3¯+3y5y2+8x3¯+5y22y=3x3+3y¯5y2+5y22y¯=3x3+y.