题目

We produce 500 billion of plastic bags in a year worldwide and they are thrown away polluting oceans, killing wildlife and getting dumped in landfills where they take up to 1,000 years to decompose．Researchers have been unsuccessfully looking for a solution. The 16-year-old Canadian high school student, Daniel Burd, from Waterloo Collegiate Institute, has-discovered a way to make plastic bags degrade（降解） in as few as 3 months, a finding that won him first prize at the Canada Wide Science Fair, a ＄10,000 prize, a ＄20,000 scholarship, and a chance to revolutionize a major environme如图所示的抛物线是二次函数y=-x2+ax+a2-4的图象，那么a的值是    ．