I have a Question < X | P | X > = ?

I have a Question < X | P | X > = ?

The bra of x and the momentum operator acting on the ket of x

i have a problem i have to prove that

< psi | P(momentum operator) | psi > =integral of the conjugate of psi * -ihd/dx *psi dx

but i have to prove first that p(operator)=ihd/dx, using the the completness theorem for posicion and momentum.