Y axis 3d rotation of matrix in javascript (matrix is of form [x1,y1,z1,x2,y2,z2,...])

mat_rotate.js

/*                                                                                                                                    
  Multiply matrices of 3d coords                                                                                                      
  multiply_matrix_3d(mat1,mat2) =                                                                                                     
  [a,b,c,a2,b2,c2,a3,b3,c3] * [x,y,z,x2,y2,z2,...]                                                                                    
 */
function multiply_matrix_3d(mat1,mat2){
    var num;
    for(var i = 0; i < mat2.length / 3; i++){
        for(var j = 0; j < 3; j++){
            // initialize                                                                                                             
            num = 0;
            for(var k = 0; k < 3; k++){
                num += mat1[3*j + k] * mat2[3*i+k];
            }
            mat2[3 * i + j] = num;
        }
    }
}

function rotate_y(theta,mat){
    // Build a rotation matrix                                                                                                        
    var r = [
	Math.cos(theta),0,Math.sin(theta),
        0,1,0,
        (-Math.sin(theta)),0,Math.cos(theta)
    ];
    // Rotate                                                                                                                         
    multiply_matrix_3d(r,mat);
}

rotate_y(0.8,vertex);