Explain your answer please.... I would like to know how to solve it myself

Consider the following set of equations:

Equation M: y = x + 4
Equation P: y = 3x + 6

Which of the following is a step that can be used to find the solution to the set of equations?
x = 3x + 6 x + 4 = 3x + 6 x + 6 = 3x + 4 x = 3x