/sci/ - Science & Math

How can I show that if [math]F : \mathbf{RMod} \to \mathbf{SMod}[/math] is any functor which preserves all colimits, then it is right exact? Specifically I'm having trouble showing exactness at [math]FB[/math] in [math]FA \overset{F\varphi}\to FB \overset{F\psi}\twoheadrightarrow FC \to 0[/math]