@111353 you’re welcome 
To answer your question about merging the two layers its summed and then the result is offset by the bias. [source: Stanford CS convnets 20] (Also a great place to learn more!)
Although I have seen some literature do an average.
@harsha_g is right, I made a slight mistake with the output!
The output of the conv operation is not the size of the input image
We can calculate the change in dimensionality from the CONV operation using this equation:
(𝑊−𝐹+2𝑃)/𝑆+1
𝑊 is the input volume
𝑃 is padding
𝐹 is filter size
𝑆 is stride