What mathematical equation can be used to represent the output of this differential amp circuit?
As a plan of attack, I would say:


Calculate \$V_+\$, which is independent of the rest of the circuit.
By the ideal op amp rules, \$V_-\$ must be equal to \$V_+\$.
(a) Write a KCL equation for the \$V_-\$ node, or...
(b) Simplify the circuit by finding the Thevenin equivalent of V, R1, and R3 from the perspective of the \$V_-\$ node.