How do I do this question?

I would use the given information to find the height of the triangle using A = bh with b=71 cm.
Then I would use that information to divide the triangle into 2 right triangles and use trigonometry to work out the angle measurement.

What Khan Academy course is this from?

Hi RJ, I'd encourage asking this question in the Math discussion forum under the relevant course video/article in Geometry or Trigonometry, or Grade Level or Get Ready Math. Yes, the discussion forum there will likely not allow for uploading pictures but I believe so long as you specify the problem clearly you should be able to get the help you need.

If you'd like live tutoring help, check out Schoolhouse.world - also founded by Sal Khan, and where you can get certified in Khan Academy Math and Science courses as well.