WASHINGTON (AP) -- He was a first-term senator-turned-president, a former law professor with little experience in economics or management. When he entered the White House he had one essential task: piece together the shards of a shattered U.S. economy. It wasn't smooth and it wasn't fast. But President Barack Obama will leave behind, by most measures, an economy far stronger than the one he inherited. Unemployment is 4.6 percent, a nine-year low. Stocks keep setting highs. An additional 20 million Americans have health insurance coverage. The nation has shifted toward cleaner energy sources: natural gas, wind and solar.