The first question: the calculation method using Thevenin theorem is correct.

Question 2: symmetrical triangle load, line voltage = phase voltage, line current =√3× phase current, and phase current is 30 ahead of line current.

The line current IA (phasor) is equal to 5 ∠ 10, so the phase current Ia (phasor) is equal to 3× 5 ∠ (10+30) = 5 ∠ 3 ∠ 40 (a).

Phase voltage Ua (phasor) =UAB (phasor) = 380 ∠ 85 V.

So: Z=Ua (phasor) /Ia (phasor) = 380 ∠ 85/5 ∠ 3 ∠ 40 = 44 ∠ 45 = 44× (∠ 2/2+j ∠ 2/2) = 3.