I will choose the bottom node as the reference node and assign node voltages V_A, V_B, and V_C to the remaining nodes.

Node A forms a supernode with the reference node due to the 4V source (shown with a dotted outline):

V_A = 4

Define v_x (used to define the dependent source) in terms of node voltages.

v_x = V_C - V_A = V_C - 4

KCL at node B:

Simplify and collect terms

- 21 - V_B + 3 V_C - 3 V_B = 0

- 4 V_B + 3V_C = 21 (Eq. B)

KCL at node C:

Simplify and collect terms.

V_A - V_C + 2 V_B - 2 V_C + 4 V_C - 16 = 0

4 + 2 V_B + V_C = 16

2 V_B + V_C = 12 (Eq. C)

Solving Equations B and C yields:

V_A = 4 V
V_B = 1.5 V
V_C = 9 V

But

v_o = V_B
v_o = 1.5 volts