Chapter 1 - Introduction to Algebraic Expressions - 1.5 Addition of Real Numbers - 1.5 Exercise Set - Page 43: 97

-5y

We know that like terms must have the same sum on both sides of the equation. Like terms must (1) be raised to the same power and (2) consist of the same variable (i.e. x and y are not like terms). It is also important to note that numbers are all like terms with each other when not multiplied by anything. Thus, combining like terms, we obtain: $$ k -2y=-7y \\ k=-5y$$