 One, two, three, four needs to be done to dismantle the system. U2 is also spherical charges. A spherical shell of charge uniformly distributed. Even though it is not a point charge, outside itself it will behave like a point charge only. So write down potential energy, trivial charges. There is one sphere is like this and other sphere is like that uniformly distributed. So I am saying that they are charged or they are insulated. Charges cannot move itself. Once you fix the charge at a location, it will remain there only. So if q1 is this q1, q2 by r, and this goes to 0.1, now 10r minus x is equal to plus minus root 2x. So x will be equal to 10r divided by 1 plus minus root 2. Now tell me 1 plus root 2. Correct. W is equal to u2 plus minus u1 plus k1. Now write down potential energy. k small q minus, then minus k into q small q, which is 9r. It is not changing before and after. So when you subtract that u2 is okay. Only x, x is wrong center. Then minus of k to q, q divided by u is understood.