Δ ABC is an isosceles triangle in which the two base angles are equal. A such 3x = 75°.
Now, since the sum of the angles in a triangle is 180°, then (4y + 10)° + 75° + 3x = (4y + 10 + 75 + 75)° = 180° ⇒ (4y + 10 + 75 + 75)° = 180° 4y = 180° - 160° y = 5°