By definition, the sum of the interior angles of a triangle is 180 degrees.
In this case, you have the triangle ABC shown in the picture. Since you know the measure of two angles (angle B and angle C), you can set up the following equation:
[tex]\begin{gathered} m\angle A+m\angle B+m\angle C=180\degree \\ m\angle A+100\degree+47\degree=180\degree \end{gathered}[/tex]Finally, you need to solve for the angle A in order to find its measure. You get that this is:
[tex]\begin{gathered} m\angle A+147\degree=180\degree \\ m\angle A=180\degree-147\degree \\ m\angle A=33\degree \end{gathered}[/tex]The answer is:
[tex]33\degree[/tex]