Question

He has been doing it. 3hrs

Answer 1

Passive Voice of this sentence is not possible
this is the right answer
hope it helps you
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Answer 2

$\huge{\bold{\red{Heya!!!}}}$


$\bold{\blue{Here\:is\:your\: answer\: }}$

It can't be changed because it is a sentence of present perfect continuous Tense.
And it is rule that we can't change the voice of any perfect continuous tense.

$\bold{\green{✓✓✓✓✓✓✓✓✓}}$


$\bold{\red{Hope\:it\:will\:help\:you\:}}$