f(x)=x²+2ax+3
f(a+1)=(a+1)²+2a(a+1)+3=a²+2a+1+2a²+2a+3=3a²+4a+4
f(a)=a²+2a²+3=3a²+3
f(a+1)-f(a)=(3a²+4a+4)-(3a²+3)=4a+1=9
解得a=2
f(x)=x²+2ax+3
f(a+1)=(a+1)²+2a(a+1)+3=a²+2a+1+2a²+2a+3=3a²+4a+4
f(a)=a²+2a²+3=3a²+3
f(a+1)-f(a)=(3a²+4a+4)-(3a²+3)=4a+1=9
解得a=2