1) mgl^2/K q ( K = 1/4pi E) its just the force of repulsion = wt of bob
2)if we didnt assume that x<<l, the equation would bcome
q^2/{(l+x)^2 (4 pi E0)} = Kx
as solution is bcming tough ,we assumed that x<<<<l, and wrote it as
q^2/{l^2 (4 pi E0)} = Kx now we get the solution easily......