This is obviously easy because it was my second question on a GMAT prep test, but to me the chart is not self explanatory. Can someone explain? Answer is C

3) What is m+n?
a. 3
b. 4
c. 5
d. 6
e. 7

+ x y z
4 1 -5 m
e 7 n 10
f 2 -4 5