1
(i) b=a+3 maps {2,3,4} in A (all of A) to {5,6,7} in B. It doesn't matter that 2 in B is not mapped.
(ii) b=a+2 maps 3 and 4 in A to 5 and 6 in B, but 2 does not map to B,
(iii) a=b-3 maps 5, 6 and 7 in B to 2, 3 and 4 in A but fails to map 2.
2 This function implies that x=f(x,y), which implies x=f(f(x,y),y), which implies x=f(f(f(x,y),y),y) and so on. The "function" is mapping back into itself iteratively, recursively and indefinitely. It cannot be a function.