Question 107784
An example of combining like terms: x+x=2x
x could equal 10.  It is the same as saying 10+10=2(10).
An example of unlike terms are x and y.  You cannot combine x and y unless the values for x and y are given.  
For example, if x=2y and y=10, then x+y = 2y+10 by the substitution property of equality.  
y=10, so 2y+10 = 2(10)+10 
                 20+10=30

However, if you were not given x and y values, then you could not combine the terms because they are not alike.  x+y cannot equal 2xy.