Its an addition table. When you add the column and row, the value in the corresponding cell is their addition.

Start with 4 and x.
The table implies 4 + x = 1. So x = -3
e + x = 7. e = 7-x = 10
f + x = 2. f = 2-x = 5

f + z = 5. z = 5 - f = 0
z + 4 = m. m = 0 + 4 = 4

f + y = -4. y = -4 -f = -9
e + y = n. n = 10 - 9 = 1

m + n = 4 + 1 = 5

Thanks givemeanid. I had no idea what the question was. But your explanation helps.

slightly shorter route:

first i found the variables needed to get the answer (m, n) and their relevant equations:
z + 4 = m
y + e = n

so z + 4 + y + e = m + n
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then all i need is z, e, and y
just pick the ones you need
z + e = 10
y + 4 = -5

plug in 10 for (z + e) and -9 for (y)

10 - 9 + 4 = m + n = 5