lomkes

how should I define substrate changing with time in michaelis menten kinetics?? I found this : S(t)=S(0)*(1-(vmax*S(0)/Km+S(0))/S(0))^t, where S(0)-initial substrate concentration, but my tutor says it's not right else I tried to solve: dE/dt = -k1*E*S + ki*ES + k2*ES dS/dt = -k1*E*S + ki*ES dES/dt = k1*E*S - ki*ES - k2*ES dP/dt = k2*ES I solved those in maple, but I get only plots(which seems fine) and no functions. So my question is what is the formula that I could use to find out how substrate concentration changes in time???