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If the parabola and the line above intersect at two distinct points, then we have the quadratic equation:
x 2 + ( a − 1 9 ) x + ( b + 1 ) = 0 ⇒ x = 2 ( 1 9 − a ) ± ( a − 1 9 ) 2 − 4 ( 1 ) ( b + 1 ) = 2 ( 1 9 − a ) ± a 2 − 3 8 a + 3 6 1 − 4 b − 4 = 2 ( 1 9 − a ) ± a 2 − 3 8 a − 4 b + 3 5 7 .
If x = 9 − 3 is one of the x − coordinates of intersection, then we require:
2 1 9 − a = 9 ⇒ a = 1 ;
a 2 − 3 8 a − 4 b + 3 5 7 = 1 2 ⇒ b = 7 7 ;
in order to attain x = 9 ± 3 as the two distinct x − coordinates of intersection. Hence, a + b = 7 8 .