SOLUTION TO Q.67
see to solve this question u must know the expression for force experienced by a dielectric slab kept inside a capacitor. Suppose a capacitor with length and width of the plated l and b respectively. the seperation between the plated is d. now a dielectric slab of dielectric constant K is kept between the plated of the capacitor such tht x length is inside the capacitor and the width of the dielectric is also d. then the dielectric is pulled inside the capacitor with a force given by
}{2d})
now let the length of the dielectric be 2x so tht x length remains inside each capacitor. now for the slab to be in equilibrium the forces exerted by both the capacitors must be equal in magnitude. applying the above formula
}{2x} = \frac{\epsilon_{0} b{E_{2}}^{2} (K_{2} - 1)}{2x})
solving this u will get

i will soon post solution to q.68 coz. its very tedious to type the complete solution here.
PLZ... RATE ME IF U UNDERSTOOD THE SOLUTION CLEARLY OR IF IN DOUBT U CAN ASK FURTHER.