Answer:
The value is placed in system of equation is -10.
Explanation:
Given : System of equations
and

To find : Which value when placed in the box (?) would result in system of equations which infinitely many solutions ?
Solution :
For a system of equations to have infinitely many solutions,
The equations have to be linearly dependent i.e. one of the equations is a multiple of the other equation.
In the given system of equations,
The second equation have to be a multiple of the first equation.
Re-write equation 1 as
The second equation is the first equation multiplied by 2,
i.e.


On comparing the missing term is -10.
Therefore, the value is placed in system of equation is -10.