The value of x + y is -16.
If each term is calculated by multiplying the previous one times 5 and then subtracting 20 from that result, then we can use that information to relate the first term (x) and the second one (0).
Following the info above, the following equation applies:
0 = 5 x - 20
then we solve for x:
5x = 20
x = 20/5
x = 4
Now we do the same study relating the second term (0) and the third one (y):
y = 5 (0) - 20
y = -20
Now that we have the value for both unknowns, we can estimate what they ask you: The value of x + y:
x + y = 4 + (-20) = 4 - 20 = -16