The word averiguasteis is the preterite form of averiguar in the second person plural.
See the full averiguar conjugation

averiguar

¿Qué averiguasteis en el vertedero?
What did you find out at the landfill?
Así que, averiguasteis lo que hacíamos los cuatro, ¿y luego qué?
So you figured out what the 4 of us did, and then what?
¿Qué averiguasteis en el extranjero?
What do you got from overseas?
Hacedme saber que averiguasteis.
Let me know what you find out.
¿Averiguasteis quién lo hizo?
Hey. You find who did it?
¿Averiguasteis algo sobre la víctima, chicos?
You guys find out anything about the victim? Yeah.
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