Add updated video links (#1041)

Author: siwelwerd

source/linear-algebra/source/03-AT/03.ptx

@@ -824,21 +824,16 @@ Are the row space and column space of <m>N</m> both equal to <m>\mathbb{R}^3</m>
     <subsection>
         <title>Videos</title>
         <figure>
-            <caption>Video: The kernel and image of a linear transformation. 
-            Note that there is a typo: if you're following along, you should find that <m>T\left(\left[\begin{array}{c}2\\1\\3\\0\end{array}\right]\right)=\left[\begin{array}{c}14\\-5\\9\end{array}\right]</m>.</caption>
-            <video xml:id="video-AT3-1" youtube="FGyD1KLFHwc"/>
+            <caption>Video: The kernel and image of a linear transformation.</caption>
+            <video xml:id="video-AT3-1" youtube="9vLnq3jywd8"/>
         </figure>
         <figure>
-            <caption>Video: Finding a basis of the image of a linear transformation</caption>
-            <video xml:id="video-AT3-2" youtube="ut_1dVFqwXw"/>
-        </figure>
-        <figure>
-            <caption>Video: Finding a basis of the kernel of a linear transformation</caption>
-            <video xml:id="video-AT3-3" youtube="VO2bDSiwbJM"/>
+            <caption>Video: Finding a basis of the kernel and of the image of a linear transformation</caption>
+            <video xml:id="video-AT3-2" youtube="Tk7OpBYQ7ps"/>
         </figure>
         <figure>
             <caption>Video: The rank-nullity theorem</caption>
-            <video xml:id="video-AT3-4" youtube="A0RzdY_g44Y"/>
+            <video xml:id="video-AT3-4" youtube="YhJdC82gQcM"/>
         </figure>
     </subsection>
 

source/linear-algebra/source/04-MX/03.ptx

@@ -368,16 +368,10 @@ but where inputs and outputs are all given in <m>\mathcal B</m>-coordinates?
 </subsection>
     <subsection>
         <title>Videos</title>
-            <p>
-Video coming soon to 
-<url href="https://www.youtube.com/watch?v=kpOK7RhFEiQ&amp;list=PLwXCBkIf7xBMo3zMnD7WVt39rANLlSdmj">this YouTube playlist</url>.
-            </p>
-        <!-- <figure>
-            <video xml:id="video-MX3-1" youtube="an-Qo2QEyXU"> 
-              <track kind="subtitles" marker="US English" xml:lang="en" source="videos/E1-captions.vtt" default=""/>
-             </video>
-            <caption>Video: Invertible matrices</caption>
-        </figure> -->
+        <figure>
+            <video xml:id="video-MX3-1" youtube="pGUyYyfbctY"/> 
+            <caption>Video: Change of basis</caption>
+        </figure> 
 
     </subsection>