.getElementById("reply_quest_4").innerHTML = "It has intersections, but not a solution. Please try again.";
    } else {
        document.getElementById("reply_quest_4").innerHTML = "Yes! The solution is when all the three lines intersect in the same point.";
        document.getElementById("buttons_quest_4").style.display = 'none';
        quests[3] = 1;
    }
    draw_plot_named('system_2', 0, 0, false);
}

function almost(a, b) {
    return Math.abs(a-b) < 1.e-5;
}

function done_system_3_quest_5() {
   