3-Phase 10kVA Y-Δ Transformer: Required Supply Voltage for 230V Load, 0.9 PF Lagging

*3) A 3-phase 230 V, 9 kVA, 0.9 PF (lagging) load is supplied by 10 kVA, 1330/230 V, 60 Hz three-phase transformer connected in Y-Δ. The equivalent impedance of transformer referred to the low-voltage side is 0.12 + j0.25 Ω. Determine the required supply voltage if the load voltage is 230 V.

This is why you are in school - for you to solve this. But I will give you a clue. Because we assume it is a balanced load - using power equations you can find the equivalent phase impedance of the load. Then you put this found impedance in series with the give equivalent transformer impedance you then can work backwards to find the required input voltage to give the desired output voltage. Drqw this out and you will see that you will have an equivalent series circuit. By the way you have to convert the transformer impedance to the high side so you can deal with the wye connection instead of the delta. You are looking for the primary voltage anyway.

By the way - what book are you using ???

If you are so desperate to find your answer on internet then maybe you must not sleep at school or read your books.