f(x)=kx²+2x-3
=k(x²+2x/k+1/k²)-3-1
=k(x+1/k)²-4
对称轴为 x=-1/k
则-1/k=1
k=-1
f(x)=-x^2+2x-3
f(2)=-2^2+2*2-3
=-4+4-3
=-3