You are told that the drug is eliminated according to the law of exponential decay so you know the form of the function, but first you need some notation. Let t be the time in hours and say t = 0 at 6am. Let D(t) be the amount of the drug, in milligrams, remaining in the body at time t. Then you know that
D(t) = A e-kt
where A and k are constants.
You are asked to find the value of D(t) at noon that is at t = 6 hours. To do this you need to determine the values of A and k. You know two facts
At 6am the amount of drug in the body is 50 milligrams,
that is D(0) = 50
At 8am the amount of drug in the body is 60% of 50 milligrams,
that is D(2) = 0.60 50
Use these facts to evaluate A and k and then find D(6).