在直角坐标系中,抛物线 y= x 2 +mx- 3 4 m 2 (m>0)与x轴交于A,B两点.若A,B两点到原点的距离分别为OA,OB,且满足 1 OB - 1 OA = 2 3 ,则m的值等于______.