Since the side lengths of the hexagon are all equal, it is a ‘regular’ hexagon. Therefore all the interior angles of the hexagon are equal.
To find the sum of the interior angles of the regular hexagon, you can use the formula
Where n is the number of sides, six in this case.
The angle you’re trying to find is labeled in red, circled by black.
You could find it by subtracting the right angle from the interior angle.
Or you could look at the angle on the left, which is an interior angle and recognize that it is the largest angle in an isosceles triangle.
From there you could subtract the interior angle from the total of 180 degrees for a triangle and then divide by two to find one of the two congruent angles.