When you work with arithmetic operations you have to pay attention to certain points. See:
var alunos = new Array();
alunos[0] = new Array();
alunos[0] [0] = "Aline dos Santos";
alunos[0] [1] = "6,5";
alunos[0] [2] = "2,9";
alunos[1] = new Array();
alunos[1] [0] = "Bianca da Silva";
alunos[1] [1] = "7,8";
alunos[1] [2] = "8,6";
var medias = new Array();
medias[0]=((alunos[0][1]+2)*alunos[0][2]/3);
alert(medias[0]);
Your code is dealing with strings, not numbers. So I ask you for your greater understanding to look for more and to know more and more. Small mistakes are for growing up, or something like that!
Code working:
var alunos = new Array();
alunos[0] = new Array();
alunos[0] [0] = "Aline dos Santos";
alunos[0] [1] = 6,5; // Retirei aspas
alunos[0] [2] = 2,9; // Retirei aspas
alunos[1] = new Array();
alunos[1] [0] = "Bianca da Silva";
alunos[1] [1] = 7,8; // Retirei aspas
alunos[1] [2] = 8,6; // Retirei aspas
var medias = new Array();
medias[0]=((alunos[0][1]+2)*alunos[0][2]/3);
alert(medias[0]);
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