The total reactance across a circuit with a resistance, an inductance and a capacitance in series is given by:

V_RLC = V_R + j(V_L − V_C) (from the definitions)

Now

|V_R | = 12 V

|V_L | = 15.5 V

|V_C | = 10.5 V

Now V_RLC = V_R + j(V_L - V_C)

= 12 + j(15.5 - 10.5)

= 12 + 5j V

The magnitude of this voltage is:

|12 + 5j| = 13 V (from calculator)