dx1/dt = u1 - k2*x1 - k1*x1;
dx2/dt = k5*x4 - k3*x2 - k6*x2 + k1*x1 - k7*x2;
dx3/dt = k3*x2 - k4*x3 + k2*x1; 
dx4/dt = -k5*x4 + k6*x2;
y1 = s2*x2; 
y2 = s3*x3
