 Solution

On shell, the contact vertex is $4i{\lambda }^2{m}^2$. At a cubic vertex with two external on-shell legs and internal momentum $k$, the rule reduces to $2i\lambda (k^2-m^2)$. The propagator cancels one such factor, so the three exchange diagrams sum toFor equal-mass two-to-two scattering, $s+t+u=4m^2$, and the exchange sum is $-4i{\lambda }^2{m}^2$. It cancels the contact diagram exactly. Thus the connected on-shell amplitude vanishes, as required by the equivalence theorem for field redefinitions: an invertible local field redefinition cannot turn a free theory into a physically interacting one.