The largest number is 2, and it's in the upper left. It got there because matrix[0][0] was incremented by 1 in both the operations.

Let's take a step back: so how do numbers in the matrix increment? It's via the overlap of the operations! What do we mean by this? The first operation gave us:

xxxxxxxxxx

[[1, 0, 0, 0],

 [0, 0, 0, 0],

 [0, 0, 0, 0]]