like terms are term that has the same variable
Ex: 5a and 19a are like terms because they have they same "a" varaible
You can simplify like terms only by add or subtract the numbers like terms
Ex: [tex]2d + 3g + 4g - 5s+9d[/tex]
in this case, [tex]3g[/tex] and [tex]4g[/tex] are like term so we add them together, 4+5= 9, so [tex]9g[/tex], is simplified answer.
Then [tex]2d[/tex] and [tex]9d[/tex] are like terms again so we also add them together, 2+9 = [tex]11d[/tex]
[tex]-5s[/tex] is alone so you just keep it the same, remember your negative sign.
The answer will be: [tex]9g + 11d - 5s[/tex]
Hoped it helped
