did you only quilt the panel? without trimming, squaring it up first? did not add any borders or blocks? if that is the case about all you can do now is choose a line and trim, square it up- hopefully without making the center appear 'wonky'. and before you quilt the next one clean up the edges and square it up the way you want it- leaving a bit on the outside to use to square up your quilted quilt- since the act of quilting causes some distortion. cutting off the cream and making the black square is probably your best bet at this point- then if one side winds up being a bit narrower than a different side it will not be very noticeable-if at all. do remember for future projects- your quilt top (doesn't matter if it's a big quilt, a wallhanging or a placemat) should be squared & even before quilting.