Find Thevenin equivalent voltage - Circuit Analysis
With such simple circuit featuring 3 sources, I would apply superposition to determine \$V_{th}\$: calculate \$V_{th1}\$ for \$V_1\$ and \$V_2\$ reduced to 0 V (shorted) while \$I_1\$ is alive, then \$V_{th2}\$ for \$I_1\$ (0 A, open) and \$V_2\$ off (0 V, short circuit) \$V_1\$ is alive and finally \$V_{th3}\$ for \$I_1\$ (0 A, open) and \$V_1\$ off (0 V, short circuit), \$V_2\$ is alive. The Thévenin voltage you want is the sum of these three intermediate steps.