Mis hermanos y yo (guardar) cama ayer.

First, let's see what person and tense is needed:

mis hermanos y yo - my brothers and I - this is equal to "we"; first person plural. 

Ayer means "yesterday" - so we need the simple past tense. 

Simple past tense of regural -ar  verbs is -amos, just like the present tense!

So the correct answer is "guardamos". 

Let me see the picture