a + b = 5c and ab = –6d c + d = 5a and cd = –6b ⇒ ac = 36 Now a satisfies first equation and c second one, so a2−5ac−6d=0 & c2−5ac−6b=0 adding these 2 are get (a+c)2−12ac−24(a+c)=0 ⇒ (a+c)2−24(a+c)−12(36)=0 ⇒ a+c=36ora+c=−12(a=c=−6) hence a + c = 36 so b + d = 144