lets say we found the accurate momentum of an electron, say p that is there is no uncertainity in this value and hence p =0. now we have =h/p , since p is definite hence the wavelengh will also b definite(we r considering the wave nature of electron) now this wave will extend all over space which means we cannot determine the position of electron with accuracy it can b at any point in the space and hence is uncertain. x=infinity. since generally the wavelenght associated wid the electron is not single hence from debroglie relation there is also some uncertainity in momentum,  and x is not equal to infinity. the relation established between the two is xp h/2