∵ Mirror image of P(a, 6, 9) is Q(20, b, –a –9). So midpoint of PQ ( i.e ,(2a​+10,2b​+3,−2a​)) lies on the given line 72a​+7​=52b​+1​=−9−2a​−1​⇒a=−56 and b = – 32a + b = – 88|a + b| = 88