How do I find the missing equation?
There are four nodes in the circuit, and one has been designated as the reference node (ground). So, you need to write KCL equations at each of the remaining three nodes.

Since you have a voltage source in the circuit you can't write KCL equations at the two nodes connected to the voltage source. Instead you must treat the voltage source as a supernode and write a single equation for the sum of currents into the two nodes of the supernode. It's this equation that you lack.

You then need an equation for the voltage between those two nodes, which you already have.