How would one summarize chapters 11 and 12 of Gary D. Schmidt's novel Trouble?

To write a summary, we focus on the most important details.

In Chapter 11 of Gary D. Schmidt's Trouble, the most important details concern the fact that Henry faced conflict from his family for his plans to travel to Maine to climb Katahdin yet left without his family's permission regardless. In the minds of his family members, Henry going off by himself to climb a dangerous mountain he would have been climbing with Franklin is just inviting more Trouble into the Smiths' home. In addition, his mother can't bear the thought of the family being separated after Franklin's death:

It's not the time for us to be splitting apart. (p. 146)

Yet, in Henry's mind, the Smith family has already split apart. Therefore, at the start of July, Henry sneaks out of the house with Black Dog on a leash and meets up with Sanborn to hitchhike to Maine. It's not until dark that Henry and Sanborn are picked up by a truck, and when they get in, they discover they have been picked up by Chay Chouan, who is responsible for Franklin's death and has just left his family out of rebellion against his father.

One important detail in Chapter 12 is that tensions between Chay and Henry begin to resolve somewhat during their conversation in the truck. Chay speaks of how sorry he is for being responsible for Franklin's death. Tensions begin to resolve the most when Chay expresses how much he is aware of Henry's mother's suffering because he witnessed his own mother suffer in the exact same way. Chay continues to relay his experience in the refugee camp in Cambodia when the Khmer Rouge came to collect boys to fight. The soldiers chose Chay's ten-year-old brother because he was large. Chay further relays that his mother begged the soldiers not to take him, and when she refused to cease begging, "a soldier shot [Chay's] sister." It is because Chay has heard his own mother grieve for her lost children that he can tell Henry, "That is how I know what your mother sounds like" (p. 159). Though Henry begins to feel compassion for Chay, tensions rise again toward the end of the chapter when Henry reflects to himself in the chowder house that he is sharing a table and dining with his brother's murderer.