1. The problem statement, all variables and given/known data This circuit solves some differential equation, the question is asking for the equation based on the circuit diagram imgur link: http://i.imgur.com/b1npQSK.png 2. Relevant equations 3. The attempt at a solution I refer to the input terminal for -f(t)...the signal from it doesn't seem to pass through any resistor on its way into the inverting summer, so I've no way (I think) of finding how the signal from -f(t) is scaled in the summer. Is this a mistake on the diagram? If not, what am I missing?