Merge 2 images with specific top, left, width, height in GM Imagemagick Nodejs

I have two single images, background.png and tree.png. I'd like to place the tree.png on top of my background.png at (150, 150 ) while resizing the tree.png to specific 100x300px. Here is what I've been able to do so far:

gm = require( 'gm' );

gm()
    .in( '-page', '+0+0' )
    .in( 'public\\media\\background.png' ) //Load the background
    .in( '-page', '+150+150' ) //X and Y position of the tree
    .in( 'public\\media\\tree.png' )
    .mosaic()
    .write( 'result.png', function (err) {

       if ( !err ) console.log( err )

    });

I am able to produce the the picture that contains the tree on top of the background at specifically 150 150. However, I don't know how to resize the tree.png to 100x300px. Please advise.

Also, is this the correct way of merging 2 layers? I did a lot of research and this one turned out to be the only one that worked for me. I'd like to hear your opinions.

Thank you so much.