Since d divides both a and b, there are integers k and l s.t.
a = kd and b = ld.
Thus a^2 + b^2 = (k^2+l^2)d^2
and a+b = (k+l) d
Thus, the gcd of a^2+b^2 and a+b is at least d, which is a)
Now, for b), we assume that a and b are both odd.
Then k and l are odd, too.
Thus k^2+l^2 is even (odd + odd is odd and odd + odd is even)
and k+l is even.
Thus 2d divides the gcd of a^2+b^2 and a+b in this case.