Solution:
The two lines have infinitely many Solutions, then the lines overlap at every point
To have infinitely many solutions, the lines must be identical consisting of exactly the same points to make it
Which means, in simplest form their equations are the same
x + y = -1
Multiply by 2
2x + 2y = -2
Thus, equation for line P is 2x + 2y = -2
Now we have,
Line M : x + y = -1
Lin P : 2x + 2y = -2
Similarly, we can obtain many equations for line P so that set of equations has infinitely many solutions