Several students had some difficulty with the problems on the homework that had voltage sources in the feedback loops of op amps. After one student explained to me his way of thinking about the voltage sources, I have some comments that I hope will help resolve the difficulties. I will use this circuit as my example:

It seems that many students have the idea that voltage sources "put" their values onto the attached nodes. In my circuit here, for example, the 3-V source puts a value of 3 onto the wire above it, and that same value propagates up to the other elements. Or, in other words, it puts a 3 onto node a.

This can be a useful way to think about voltage sources, and we can get correct values of node voltages this way, as long as we do it right. The problems come in when we have two voltage sources connected to the same node, and the values they put don't match:

Here we have the 3-V source putting a 3 onto node a, and the 2-V source putting a 2 onto the same node. What went wrong?
To resolve the problem, we need to remember that the value of a voltage source is the difference between the potentials at its terminals. That means that we cannot use the value to determine the node voltage at one of its terminals until we already know the node voltage at its other terminal. Let's try the example problem again, keeping this fact in mind.
First, we see that node c is grounded, so its node voltage is 0. Now that the node voltage at one terminal of the 3-V source is known, we can use the value to find the node voltage at the other terminal:

The node voltage propagates along all the wires that make up node a, up to the terminal of the 2-V source. Given a's node voltage, we use the value of 2 V to find b's node voltage:

In summary, remember that the values of voltage sources are the differences between the node voltages at their terminals.