Theorem 2: If both f:X-Y and g: Y→ Z then the composition fog is also one-to-one. are one-to-one fonctions

Let x=x(t) be a twice-differentiable function and consider the second order differential equation x+ax+bx=0(11) Show that the change of variables y = x' and z = x allows Equation (11) to be written as a system of two linear differential equations in y and z. Show that the characteristic equation of the system in part (a) is 2+a+b=0.