Added method to solve linear, 1st order and 2 eq system

This PR contain method to solve "System of two linear homogeneous first-order constant-coefficient ordinary differential equations". It solved by differentiating first equation and replacing the value of y' from second equation to the equation obtained. The second order equation is obtained on x(t) which is then solved and the value of x(t) is put on second equation thus obtaining a first order equation in y(t). This equation upon solving give value of y(t).: