The sum of angles in any triangle is 180° So the missing angle in the bottom triangle is:
180-76-54=50°
Now that right angle (the square) is bisected by this 50° so the bottom angle of the inverted upper triangle is 90-50=40°
And again, because the sum of angles in a triangle is always 180°. The missing angle in the inverted triangle the "?" is:
180-70-40=70°
So "?"=70°