You have the following equation:
-46 - 6k = -50 - 7k
In order to solve the previous equation for k, you proceed as follow:
-46 - 6k = -50 - 7k add 46 both sides and add 7k both sides
-6k + 7k = - 50 + 46 simplify similar terms
k = -4
Hence, the solution of the given equation is k = -4