Add solution

For "Use the flex-basis Property to Set the Initial Size of an Item" challenge

curriculum/challenges/english/01-responsive-web-design/css-flexbox/use-the-flex-basis-property-to-set-the-initial-size-of-an-item.english.md

@@ -74,7 +74,30 @@ tests:
 ## Solution
 <section id='solution'>
   
-```js
-// solution required
+```html
+<style>
+  #box-container {
+    display: flex;
+    height: 500px;
+  }
+
+  #box-1 {
+    background-color: dodgerblue;
+    height: 200px;
+    flex-basis: 10em;
+  }
+
+  #box-2 {
+    background-color: orangered;
+    height: 200px;
+    flex-basis: 20em;
+  }
+</style>
+
+<div id="box-container">
+  <div id="box-1"></div>
+  <div id="box-2"></div>
+</div>
 ```
+
 </section>