What conditions must the constants, the b's, satisfy so that each of these systems has a solution?

Question:

What conditions must the constants, the b's, satisfy so that each of these systems has a solution? Hint. Apply Gauss's Method and see what happens to the right side.
(a) x - 3y = b1
3x + y = b2
x + 7y = b3
2x + 4y = b4
(b) x1 + 2x2 + 3x3 = b1
2x1 + 5x2 + 3x3 = b2
x1 + 8x3 = b3