There may be a better way than this but I think you could use graph to see it:
(1) x + y ≥ a
(2) x − y ≤ −1
Hence we have
y ≥ a - x
And y ≥ x + 1
Hence y would be in the region about either line (above a-x to the left of intersection and above x+1 to the right)
a) When a<0, there is no...