No. of status posted by Ashley each day = 17
No. of status posted by Roberto each day =21
An algebraic expression to describe the combined number of posts for Ashley and Roberto after any given number of days can be written as
17x+21y
An algebraic expression to describe the difference between number of posts for Ashley and Roberto after any given number of days can be written as
17x -21y
Here in above equations, x and y represents the numbers of days for Ashley and Roberto.