Valmiki Ramayana Ayodhya Kanda Sarga 23 Shloka 12
Original Shlokaकथं त्वं कर्मणा शक्तः कैकेयीवशवर्तिनः। करिष्यसि पितुर्वाक्यमधर्मिष्ठं विगर्हितम्।।2.23.12।।
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कर्मणा by your act, शक्तः being powerful, त्वम् you, कैकेयीवशवर्तिनः who is under the influence of Kaikeyi, पितुः father's, अधर्मिष्ठम् unjustifiable, विगर्हितम् contemptible, वाक्यम् words, कथम् how, करिष्यसि can you carry out?
When you are able to counter (this move), why do you want to execute the unjustifiable and contemptible words of the king who is under the spell of Kaikeyi?