in an addition table, ech number inside the table is the sum of the headers in its column and row.
for example, m=z+4, and n=y+e.
The question is looking for m+n, or z+4+y+e. You can start finding the values of z, y and e from the table, but there's a quicker way: rearrange the sum as z+e+y+4.

z+e = 10 (go down the z column until you reach the row for e)
y+4=-5 (4 in the first row header, y in the middle column)
Thus, z+e+y+4=10-5=5.