# Ideal OP-Amp circuit

In this circuit can I set the node which has (-) colored with blue as a reference node or not? If not, is that because there are ground nodes below v1 and v2?

With considering vo= v+(blue)-v-(blue) and considering v-(blue) as a normal node (its voltage not equal to 0) I could express vo= (v1-v2)*((R1+R3)/R2 +1)

Clearly, $v_o$ is intended to be V+ - V-, and your expression is correct.