Multply alpha mask pronblem.

Hi , i have this node setup ,

i have 3 images :

one b/w image that i will use as a mask
one mounains background image
and one blender render layer

i want the blender render layer to be masked out by the b/w mask , i tried to do it via the multiply node , but is not working,

¿what im i doing wrong?

here’s a screenshot :

thanx !!!

I don’t think that you want to multiply, just use the alpha layer mask as the input for an alpha layer when you are combining or try using mix node

i cant get it , could you explain how the set up would be ?

thanx !

Add a mix node. Inputs are your two color sources (render and mountains). Attach your b&w image to the Fac (factor) input. If it looks wrong, reverse the input sources.