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Answer:(a) ∠DAB  = 59°(b) ∠A  = 135°(c)  ∠A   = 99°Step-by-step explanation:Theorem: If a quadrilateral is inscribed inside a circle, then the opposite angles of the quadrilateral are SUPPLEMENTARY.So, here in the given figures:(1) ∠DCB +   ∠DAB  = 180°  (as opposite angles are supplementary)⇒∠DAB   =  180°  -   121°   =  59°or, ∠DAB  = 59°(2) ∠DCB +   ∠DAB  = 180°  (as opposite angles are supplementary)⇒(3x+6) + (x+2) = 180° or, 4x = 180 - 8 = 172⇒ x = 172/4 = 43, or x  = 43So, ∠A  = (3x+6)  =  3(43) + 6 = 135°(3) (28) +  (x) = 180°or, x =  180 - 28 = 152So, ∠A  = (x- 36)  =  135 - 36  = 99°(4)