设f(x)=a(x-h)² + k由题可知h=1,k=4
于是f(x)=a(x-1)² + 4
把f(3)=0代入得:f(3)=a(3-1)² + 4 = 0
解得a=-1
所以f(x)=-(x-1)² + 4
=-x² +2x+3