Find real parameter $a$, such that the solution of the linear system lies in the second quadrant 
For which real parameter $a$ lies the solution of the system of equations
$$\begin{aligned} \frac{x}{a+1} + \frac{y}{a-1} &= \frac{1}{a-1}\\ \frac{x}{a+1} - \frac{y}{a-1} &= \frac{1}{a+1} \end{aligned}$$
in the second quadrant?

I do not how to start to solve this system of equations. Any help?

Solve your system : you should find 
$$x=\frac{a}{a-1},\quad y=\frac{1}{a+1}$$
You want $x<0$, so $a$ and $a-1$ must be of opposite signs. So you should have $a>0>a-1$, hence $0<a<1$, and this leads to $y>0$.
The solution is : $(x,y)$ is in the second quadrant if and only if $0<a<1$, or $0\le a<1$ if you look for the closed second quadrant.
