How to stretch the block to the height of the parent if the parent has 100VH?

Question

css

The first screen should be 100vh (it has a header + menu + first section)
How to make the block (first-screen) occupy the entire remaining height if the height of the header and menu is unknown ???

<div class="parent">
  <div class="top-line"></div>
  <div class="menu"></div>
  <div class="first-screen"></div>
</div>

.parent {
  height: 100vh;
}

Answer 1

You need to use flex-box.
And the one that should take up the remaining space flex-grow: 10 [for example].