I have to determine for which value of parameter m the system has

a) indefinite solutions

b) no solution.

System is:

2x + (m-1)y = 3

(m+1)x + 4y = -3

From first equation x = (3 - y(m-1)) / 2x and

pluging that x into second equation I have get that (if I am right)

y = (3(3+m)) / (m-9)(m+1)

In book stands that a) m = -3 and b) m = 3.

Can someone explain me is that right solution and if it is, why it is?