h1.titulo{
    text-align: center;
}

.contenedor{
    width: 960px;
    margin-left: auto;
    margin-right: auto;
    margin-top: 10px;
}

section.columna_1{
    width: 260px;
    float: left;
}

section.columna_2{
    width: 700px;
    float: right;
}

li.tres_1{
    width: 33.3%;
    float: left;
}

li.tres_2{
    width: 33.3%;
    float: center;
}
li.tres_2{
    width: 33.3%;
    float: right;
}

.regresar{
    margin-top: 850px;
    text-align: center;
    font-size: 25px;}
    .back{
        font-family: Verdana, Geneva, Tahoma, sans-serif;
        text-decoration: none;
        color: black;}
        .back:hover{
            text-decoration: underline;
}