For a system of equations to have infinitely many solutions, the equations have to be lineary dependent, (i.e. one of the equations is a multiple of the other equation). For the given equation to have infinitely many solutions, the second equation have to be a multiple of the first equation.
The first equation can be rewritten as y - 2x = -5
The second equation is the first equation multiplied by 2, i.e. 2(y - 2x= -5) = 2y - 4x = -10
Therefore, the correct answer is -10 (option a).
