Determine the number of possible routes from A to B is you travel only south or east. (in both pictures, A is a dot or point at the very top left, and B is a point or dot at the very bottom right)

My specific questions:

So in the first picture with no numbers, I'm having trouble coming up with the numbers in red in the second picture with numbers in black and red. This is the answer that I get in the back of my textbook. I understand how they get the black numbers, but the red numbers, in particular.. the 36 in red, I don't understand how they get. Please explain as best as you can thoroughly. Thanks