PINE BLUFF, Ark. — Michelle Obama told graduates Saturday to prepare to overcome adversity, building on Martin Luther King Jr.'s 1958 commencement address at the same university, when he told students to summon their courage to fight segregation. The first lady gave an impassioned speech to 270 graduates of the University of Arkansas at Pine Bluff that referenced the legacy of their historically black school, which opened in 1873 with seven students, most of whom could barely read. Obama said those first students, only a decade removed from slavery, had no guarantee of opportunity once they graduated. "Let's just imagine how those seven students would feel if they could see you here today," Obama told a packed downtown arena. She singled out Quinna Childress of Newport, who graduated Saturday with a 3.935 grade-point average in biology and plans to attend medical school. Childress was homeless at age 16, a high school student living out of a car who worked nights and weekends.