Final answer:
To combine like terms in the expression -x + 3y - 3x, we combine -x and -3x to get -4x while leaving 3y unchanged. The simplified expression is -4x + 3y since we cannot combine x and y terms.
Step-by-step explanation:
When combining like terms, we look for terms in an algebraic expression that have the same variable parts. In the expression -x + 3y - 3x, the like terms are -x and -3x.
To combine them, we just add their coefficients (while keeping the variable part the same).
-x - 3x becomes -4x, as we add (-1) + (-3) to get -4.
Therefore, the simplified expression is -4x + 3y.
Note that we cannot combine the term 3y with -4x because they are not like terms; the variable parts are different.
The variable part for -4x is x, and the variable part for 3y is y.