Answer: - 18
Explanation:
Lets just ignore the negative signs since they might make the question confusing, (You can only do this when both numbers are negative)
Y = 5, W = 31,
X has to be in the exact middle.
(Y+W) / 2 = 36 / 2 = 18
X would have to be at 18 to be in the middle point of W and Y.
We go back to the first thing we did, put the negative sign back,
X would be -18