NueveReinas

HTML

<div class="caja--roja">
      <form action="" id="formulario">
        <input type="text" class="input--grande" />
        <input type="text" class="input--pequenio" />
        <input type="submit" class="btn--busqeda" />
      </form>
    </div>

CSS

.input--grande{
   width: 100%;
   margin-bottom: 10px;
}

.input--pequenio{
  width: calc(100% - 60px);
}

.btn--busqeda{
  width: 50px;
  float:right;
}

.caja--roja{
  height: 100%;
  width: 50%;
  padding: 10px;
  background-color: #AD1B2E;
}