.container { position: relative; }
.rect { width: 334.5px; height: 223px; background: #34EFEE; text-align: center; line-height: 223px; }
.rect:after { content: ''; position: absolute; bottom: 0px; width: 334.5px; height: 15px; background: #FFFFFF; left: 0; border-top-left-radius: 50%; border-top-right-radius: 50%; }

<div class="container">
    <div class="rect">
        <h3>334.5 X 223</h3>
    </div>
</div>