see the attactment
Draw a horizontal line through point E.
This forms some right triangles with leg measures of 10 and 17 because point E is vertically and horizontally centered inside the rectangle.
Therefore, the measure of half of angle BEC can be found using the inverse tangent function with the ratio 10/17.
Double the result.
Actually, you do not need the horizontal line because half of angle BEC is the same as the measure of angle BAE. And the ratio 20/34 simplifies to 10/17, which brings us back to the same input for the inverse tangent function.