Heya!
So the key is at the top. What I mean by the Key is a=1 b=2 c=3 d=4 e=5 and f=0. You use those numbers and plug them into the associated letters in the problems below. For example, question one with the numbers in it would be, (I might be wrong on this, I apologize)
You would then solve the equation, and whatever answer you get for that equation would go with the corresponding numbers and letters on the left.
So for example, if your answer to the first one for example is 7, you would type A. You would then do the same for question 2, 3, and 4, which should reveal the four letter code for you to continue :))
Hope this helped!!