14.直线l1:ax+3y+4=0l_{1}:ax+3y+4=0l1:ax+3y+4=0,l2:x+(a−2)y+a2−5=0l_{2}:x+(a-2)y+a^{2}-5=0l2:x+(a−2)y+a2−5=0,若两条直线平行,则a=(a=(a=( )))