Step 1:

Diameter of the two spheres is .

Charge on first sphere is .

Charge on second sphere is .

Diastance between the two spheres is .

Radius of the sphere is .

Electric field strength due to the charged partical is .

Find the field strength of the first sphere .

Total distance from center of the sphere to the mid point is .

Substitute corresponding values in the formula.

.

Step 2:

Find the field strength of the second sphere .

Total distance from center of the sphere to the mid point is .

Substitute corresponding values in the formula.

.

Since moid point is considered , the magnitude of the right sphere contribution is also same as the left sphere and is in oppsit direction.

.

.

Solution :

Electric field strength at the midpoint between the two spheres is .