// returns a new matrix
Matrix.prototype.mult = function(other) {
    if (this.width != other.height) {
        throw "error: incompatible sizes";
    }

    var result = [];
    for (var i = 0; i < this.height; i++) {
        result[i] = [];
        for (var j = 0; j < other.width; j++) {
            var sum = 0;
            for (var k = 0; k < this.width; k++) {
                sum += this.mtx[i][k] * other.mtx[k][j];
            }
            result[i][j] = sum;
        }
    }
    return new Matrix(result);
}

var a = new Matrix([[1,2],[3,4]])
var b = new Matrix([[-3,-8,3],[-2,1,4]]);
print(a.mult(b));