Given:
- Two unit negative charges are placed on a straight line.
- A positive charge q is placed exactly at the midpoint between these unit charges.
- The system of these three charges is in equilibrium.

To find:
- The value of q (in C)

Explanation:
1. Understanding the problem:
- In this problem, we have a system of three charges: two unit negative charges and one positive charge.
- The charges are placed in a straight line, with the positive charge at the midpoint between the negative charges.
- The system is in equilibrium, which means that the forces acting on each charge cancel out, resulting in no net force.

2. Equilibrium condition:
- For a system of charges to be in equilibrium, the net force on each charge must be zero.
- The net force on a charge is the vector sum of the forces exerted on it by the other charges.
- In this case, the positive charge experiences a repulsive force from each of the negative charges.
- The magnitudes of these forces are given by Coulomb's law: F = k * q1 * q2 / r^2, where k is the electrostatic constant, q1 and q2 are the charges, and r is the distance between them.
- As the system is in equilibrium, the magnitudes of the two repulsive forces acting on the positive charge must be equal.

3. Analyzing the forces:
- Let's assume the distance between the positive charge and each negative charge is d.
- The magnitude of the force between a negative charge and the positive charge is F1 = k * (-1) * q / d^2.
- The magnitude of the force between the other negative charge and the positive charge is F2 = k * (-1) * q / d^2.
- Since the forces must be equal in magnitude, we have F1 = F2, which leads to (-1) * q / d^2 = (-1) * q / d^2.

4. Solving for q:
- Simplifying the equation, we get q / d^2 = q / d^2.
- Cross-multiplying, we have q * d^2 = q * d^2.
- Dividing both sides by q, we get d^2 = d^2.
- Taking the square root of both sides, we have d = d.
- The distance between the positive charge and each negative charge is the same, so d = d/2 + d/2 = d.
- Therefore, the distance between the positive charge and each negative charge is equal to the distance between the negative charges.
- Since the negative charges are unit charges, the distance between them is 1 unit.
- Therefore, the distance between the positive charge and each negative charge is 1 unit.

5. Calculating q:
- Using the distance d = 1 unit, we can find the value of q.
- Using Coulomb's law, we have F = k * q1 * q2 / r^2.
- Plugging in the values, we have F = k * (-1) * 1 * 1 / 1^2 = -k.
- As the forces must be equal, we have -k = -k.
- Therefore