Isosceles Triangle. Two equal sides. Two equal angles ; ∠ X = ∠ Z ∠Y + ∠X + Z = 180 --> 40^\circ + ∠X + ∠X = 180^\circ ∠2X = 180^\circ - 40^\circ = 140^\circ ∠X = 70^\circ, i.e ∠X = 70 and ∠Z = 70^\circ. The exterior angle of triangle is the sum of two opposite internal angles : ∠X + ∠Y = ∠XZT =70^\circ + 40^\circ = 110^\circ