28. 设二次函数f(x)=(a−2)x2+3ax+2f(x)=(a-2)x^{2}+3ax+2f(x)=(a−2)x2+3ax+2在RRR上有最大值,最大值为mmm(a),当mmm(a)取最小值时,a=(a=(a=( )))