Question from Messiah's Quantum Mechanics

The question that I don’t even know where to start on is as follows:
Utilizing the fact that any wave can be considered as a superposition of plane waves, show that in the absence of a field, the matter wave $psi(mathbf r_2,t_2)$ at the point $mathbf r_2$ at the instant $t_2$ can be deduced from the values $psi(mathbf r_1,t_1)$ taken by the wave at the instant $t_1$, by the operation $$psi(mathbf r_2,t_2) = int K(mathbf r_2 -mathbf r_1;t_2-t_1)psi(mathbf r_1,t_1) dmathbf r_1$$ where $$K(mathbf rho;tau) = (2pihbar)^{-3}int exp[frac ihbar(mathbf p*mathbfrho -Etau)]dmathbf p$$ (where $x*y$ is the dot product) an expression in which $E$, a function of $mathbf p$ is equal to the total energy of the particle, corresponding to the momentum $mathbf p$. Show that for a non-relativistic particle of mass $m$, $$K(rho;tau) =exp(-frac 34 pi i) (frac m{2pihbartau})^{frac 32}exp(ifrac{mrho^2}{2hbartau})$$ this is only part of the question, but I am clueless.