y=ax^2+4x+a
=a(x^2+4x/a)+a
=a(x+2/a)^2+a-4/a
∵y有最小值3
∴a>0
且最小值为ymin=a-4/a=3
a^2-3a-4=0
(a+1)(a-4)=0
a=-1