Final answer:
To find the sum or difference, add or subtract the coefficients of x and y, respectively.
Step-by-step explanation:
To find the sum or difference of (2x+3y) and (4x+9y), we simply add or subtract the coefficients of x and y, respectively.
For the x terms, we add the coefficients: 2x+4x=6x.
For the y terms, we add the coefficients: 3y+9y=12y.
Therefore, the sum or difference of (2x+3y) and (4x+9y) is 6x+12y.