Okay, first of all, you are probably stuck and don't understand what an exterior angle is. I don't know how to explain it but the exterior angle in this case is ∠DAB.
∠DAB = ∠ABC + ∠BCA
= 90° + 62°
= 152°
They said that the exterior angle was instead 128 degrees. This means that now the sum of the angles of B and C are 152 degrees, or, 152 - 128 = 34 degrees greater.
