Centering a div for the mobile version without using specific values for margins

Question

I have a div on top of my slider like this

In my mobile version I want to center it and i spent 2 hours trying to figure out how , without using a specific value for the margins.

The relevant css code is here

/*  Web version    */

.img-slide .advert {
    position: absolute;
    top: 60px;
    left: 70px;
    width: 610px;
    height: 170px;
}


/* mobile version */

.img-slide .advert {
        height: 120px;
        width: calc(100% - 90px);
        background-color: rgb(166 187 91 / 0.60);
    }