dx/dt = -6x-10y
dy/dt = 10x+6y
E(x,y) = 5x^2+6xy+5y^2
a. verify that E is a constant of motion for the system of the first two equations
b. find the equilibria of the system of the first two equations
c. find the critical points of E, and classify each as a max, min, or saddle point
d. classify each equilibrium of the system of the first two equations as stable or unstable